Add `repkv_backward_kernel2` and `repkv_kernel2` (llama3 branch)

[insop]

Changes

Add repkv_backward_kernel2

• improve repkv_backward_kernel1 by reducing thread used per @karpathy's suggestion

Also add repkv_kernel2 simiar to backward_kernel2

Here is the test output for repkv_backward_kernel2

# ./repkv_backward 2                                                                                              │
Using kernel 2                                                                                                                                                                    │
Checking block size 32.                                                                                                                                                           │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 64.                                                                                                                                                           │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 128.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 256.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 512.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 1024.                                                                                                                                                         │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
All results match. Starting benchmarks.                                                                                                                                           │
                                                                                                                                                                                  │
block_size   32 time 1.8824 ms                                                                                                                                                    │
block_size   64 time 0.9740 ms                                                                                                                                                    │
block_size  128 time 0.9716 ms                                                                                                                                                    │
block_size  256 time 0.9740 ms                                                                                                                                                    │
block_size  512 time 1.0151 ms                                                                                                                                                    │
block_size 1024 time 1.0725 ms 

Execution time is improved compared to kernel1 time shown below from previous PR (https://github.com/karpathy/llm.c/pull/764)

All results match. Starting benchmarks.

block_size   32 time 3.2461 ms
block_size   64 time 1.7509 ms
block_size  128 time 1.7374 ms
block_size  256 time 1.7441 ms
block_size  512 time 1.8092 ms
block_size 1024 time 2.0443 ms

Here is the test output for repkv_kernel2

# ./repkv 2                                                                                                       │
Using kernel 2                                                                                                                                                                    │
Checking block size 32.                                                                                                                                                           │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 64.                                                                                                                                                           │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 128.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 256.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 512.                                                                                                                                                          │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
Checking block size 1024.                                                                                                                                                         │
0.680375 0.680375                                                                                                                                                                 │
-0.211234 -0.211234                                                                                                                                                               │
0.566198 0.566198                                                                                                                                                                 │
0.596880 0.596880                                                                                                                                                                 │
0.823295 0.823295                                                                                                                                                                 │
All results match. Starting benchmarks.                                                                                                                                           │
                                                                                                                                                                                  │
block_size   32 time 1.7765 ms                                                                                                                                                    │
block_size   64 time 0.9856 ms                                                                                                                                                    │
block_size  128 time 0.9781 ms                                                                                                                                                    │
block_size  256 time 0.9887 ms                                                                                                                                                    │
block_size  512 time 1.0429 ms                                                                                                                                                    │
block_size 1024 time 1.1434 ms

Execution time is improved compared to kernel1

block_size   32 time 3.6582 ms                                                                                                                                                    │
block_size   64 time 1.5909 ms                                                                                                                                                    │
block_size  128 time 1.5868 ms                                                                                                                                                    │
block_size  256 time 1.5798 ms                                                                                                                                                    │
block_size  512 time 1.6164 ms                                                                                                                                                    │
block_size 1024 time 1.8981 ms