[TRTLLM-14904][fix] Work around flashinfer 0.6.15 autotuner cache-key hash/eq inconsistency

[brnguyen2]

Description

flashinfer 0.6.15's DynamicTensorSpec defines a custom __hash__ that skips tensor_initializers (and hashes callables by identity) while keeping the dataclass-generated __eq__, which compares all fields by value. Callers such as trtllm_batch_decode_with_kv_cache_mla build a fresh TuningConfig per call with fresh initializer closures, so the lru_cache on AutoTuner._find_nearest_profile accumulates hash-equal but eq-unequal keys up to its 16384 cap. Every autotuner cache probe on the MLA decode path then walks the whole collision chain in Python __eq__ — a 17–19 ms host stall per eager MLA generation call. Mixed prefill+decode iterations run eagerly and pay this on every call; decode-only iterations replay CUDA graphs and are unaffected, which made it present as a prefill-side regression. flashinfer 0.6.14 built a fresh bucket mapper per call, so keys hashed differently and cache probes missed in O(1); the 0.6.14→0.6.15 bump (#16530) exposed the inconsistency.

This PR installs a guarded TRT-LLM-side workaround where the fmha flashinfer backend imports flashinfer: DynamicTensorSpec.__eq__ is replaced with one consistent with its existing __hash__ (ignore tensor_initializers, compare callables by identity). A behavioral probe applies the patch only when the inconsistency is actually present, and any failure degrades to not patching, so a future flashinfer release with different internals is unaffected.

The bug is upstream in flashinfer; an upstream issue is being filed, and this workaround should be dropped once a fixed release is picked up.

Test Coverage

• Serving benchmark at high concurrency on a large MoE model with MLA attention: output throughput recovered ~15% (back to par with the pre-regression baseline band); low-concurrency throughput unchanged.
• CPU microbenchmark of the autotuner cache probe: ~2 µs per call with cache size 1 post-patch, versus ~10 ms per call with an 8000-entry collision chain unpatched.

PR Checklist

• PR title and description are self-explanatory
• Commit message includes ticket ID and is signed off (DCO)
• Change is guarded and degrades safely if flashinfer internals change