Improve the latency of `load_batched_adapter_weights`

[thincal]

Feature request

Currently, every lora layer would be moved from CPU to target device of base model, results in extra 20ms in each layer, finally 500ms ~ 1+s latency overall.

1. first loading into CPU memory

def load_module_map(
    ...
    for filename in adapter_filenames:
        adapter_weights.update(load_file(filename))
   ...

2. moving to GPU device inside load_batched_adapter_weights

lora_a = lora_a.to(base_device, self.dtype)
lora_b = lora_b.to(base_device, self.dtype)

Motivation

Improve the adapter loading performance.

Your contribution

Yes, I will prepare a PR for review.

[tgaddair]

Thanks for working on this @thincal! We could probably work around this by keeping weights in the safetensors file rather than loading to CPU as an intermediate step.