Update decode kernel benchmark with new Triton backend interface

[amosyou]

Motivation

The Triton kernel for decode attention was updated with a new backend interface in #3292, breaking the benchmark code.

Modifications

Corrected the import for should_use_tensor_core and replaced req_to_token, b_req_idx, b_seq_len with kv_indptr and kv_indices.

Checklist

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• [ ] Add unit tests as outlined in the Running Unit Tests.
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• [ ] Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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