Add flashinfer

[kanz-nv]

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flowchart LR
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    end
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[kanz-nv]

/ok to test 7b6bb2c

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[tdene]

@kanz-nv Would it be a good idea to split up this PR into two halves, the first one focusing on the kv_cache.py refactor, including things such as splitting up of the num_attention_heads parameter; the second one focusing on introducing new AttnBackend for FlashInfer?

[santhnm2]

@mathemakitten This will likely have significant perf implications, could you help us run RL benchmarks in addition to standalone inference tests so we avoid regressions?

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