[Perf] Delete redundant operations in model_runner and forward_context

[realliujiaxu]

What this PR does / why we need it?

Remove redundant operations from model_runner and forward_context. This optimization can significantly reduce the idle time (bubble) before decoding when running models with small parameter counts (e.g., Qwen/Qwen2.5-0.5B).

Testing on 800I A2, bubble is reduced from 3.8ms to 2.8ms :
Before

After

Does this PR introduce any user-facing change?

No

How was this patch tested?

• vLLM version: v0.11.0rc3
• vLLM main: vllm-project/vllm@17c540a

[github-actions]

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[gemini-code-assist]

Code Review

This pull request refactors the codebase by moving some checks into utility functions and removing redundant operations. The changes in model_runner_v1.py correctly fix a bug where stale data was being used and remove redundant code, improving correctness and clarity. However, the new utility function has_layer_idx in utils.py introduces a critical bug due to a flawed caching mechanism. I've provided a comment with a suggested fix for this issue.

[gemini-code-assist]

The current implementation of has_layer_idx uses a global variable _HAS_LAYER_IDX to cache its result. This caching mechanism is flawed because the function's outcome depends on the model_instance argument, which is not guaranteed to be the same across all calls.

For instance, set_ascend_forward_context can be invoked with model_instance=None (e.g., from kv_connector_no_forward), which would cause _HAS_LAYER_IDX to be cached as False. Any subsequent calls, even with a valid model_instance, would then incorrectly return False, preventing features that rely on layer_idx from being enabled.

Since this check is inexpensive, I recommend removing the caching mechanism to fix this bug. The global variable _HAS_LAYER_IDX should also be removed.

def has_layer_idx(model_instance: torch.nn.Module) -> bool:
    return (model_instance is not None and hasattr(model_instance, "model") and
            hasattr(model_instance.model, "start_layer"))

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.