Revert "[Bugfix][MoE] Unpad routed output before shared expert add [Fixes #35949]" (#40794)

[vllm-agent]

Revert of #40794

Reason: This PR introduced 3 new CI failures in build #62894:

• GPQA Eval (GPT-OSS) (B200) — RuntimeError: view size is not compatible with input tensor's size and stride in symm_mem.py:133 during all_reduce. The unpadded tensor slice (fused_output[..., :routed_hidden_dim]) produces a non-contiguous tensor that fails view(-1).
• GPQA Eval (GPT-OSS) (H100) — Same tensor view error via moe_runner.py:371 _maybe_reduce_final_output -> tensor_model_parallel_all_reduce -> symm_mem.all_reduce.
• LoRA TP (Distributed) — test_gpt_oss_lora_tp2[True-False] generates garbled output instead of SQL, indicating model correctness regression in gpt-oss MoE with LoRA in TP mode.

Root cause: The PR adds fused_output = fused_output[..., :routed_hidden_dim] which creates a non-contiguous view. When this tensor is later passed to tensor_model_parallel_all_reduce, the symm_mem communicator calls .view(-1) which requires contiguous memory. A .contiguous() call before the all-reduce would likely fix the issue.

Auto-generated by CI failure analyzer.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request removes the padding detection and unpadding logic for hidden states in the MoE runner. Feedback indicates that this revert re-introduces dimension mismatch issues between fused and shared outputs. It is recommended to retain the padding logic and apply a .contiguous() call after slicing the fused output to resolve runtime errors observed in CI while maintaining correct tensor shapes.

[gemini-code-assist]

Reverting the padding detection logic re-introduces a bug where fused_output and shared_output have mismatched dimensions when padding is applied (Fixes #35949). Instead of a full revert, these variables should be retained to allow for proper unpadding before the outputs are combined.

        # Record before `_maybe_pad_hidden_states` pads activations to match
        # `moe_config.hidden_dim`, e.g. after `align_trtllm_fp4_moe_hidden_dim_for_fi`
        routed_hidden_dim = hidden_states.shape[-1]
        hidden_states, og_hidden_dim = self._maybe_pad_hidden_states(
            shared_experts_input,
            hidden_states,
        )
        hidden_dim_was_padded = hidden_states.shape[-1] > routed_hidden_dim

[gemini-code-assist]

The RuntimeError and garbled output reported in CI are caused by the non-contiguous tensor produced by slicing fused_output. Adding .contiguous() after the slice resolves these issues while maintaining the fix for shape mismatch during the addition of shared and routed expert outputs.

Suggested change

	shared_output, fused_output = _unpack(result)
	shared_output, fused_output = _unpack(result)
	if hidden_dim_was_padded:
	fused_output = fused_output[..., :routed_hidden_dim].contiguous()

[netanel-haber]

See #40865