[TRTLLM-12339][feat] enable TRTLLM cross attention backend

[cascade812]

Description

Split out the attention operator and TRTLLM attention backend changes from #13919 to reduce frequent conflicts with main and make CI validation easier for this smaller, self-contained scope.

This PR intentionally keeps the change self-contained:

• wires thop.attention and its nanobind signature for cross-attention and relative-attention-bias inputs
• enables the TRTLLM backend path for cross-attention metadata, including Q padding, cross K/V forwarding, and beam-width handling
• makes trtllm-gen decline cross-attention so cross requests use the THOP path
• adds only the small backend forward-args fields required by the TRTLLM backend

No module, executor, model, or LLM API caller changes are included in this split.

[cascade812]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #54082 [ run ] triggered by Bot. Commit: 46ef1af Link to invocation

[tensorrt-cicd]

PR_Github #54082 [ run ] completed with state SUCCESS. Commit: 46ef1af
/LLM/main/L0_MergeRequest_PR pipeline #43166 completed with status: 'FAILURE'

CI Report

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CI Agent Failure Analysis

Link to invocation