[Bugfix] Remove incorrect assertion in causal_conv1d_update for Qwen3.5 GDN layersfix: remove incorrect assertion in causal_conv1d_update for GDN index…

[Rks2302]

Summary

Removes incorrect assertion in causal_conv1d_update that causes AssertionError
when running Qwen3.5 models with GDN (Gated Delta Networks) layers.

Root Cause

Line 1161 in causal_conv1d.py:

assert num_cache_lines >= batch

This assertion is incorrect when conv_state_indices is provided. In that case:

• conv_state is a shared cache pool, not a per-batch tensor
• num_cache_lines is the state length (typically 4-6), NOT the batch dimension
• The actual batch validation is already correctly handled in the if/else block above

The correct validations are already present:

• Without indices: assert conv_state.size(0) >= batch ✅
• With indices: assert batch == conv_state_indices.shape[0] ✅

The removed assertion is therefore redundant and incorrect.

Error Reproduced On

• GPU: NVIDIA RTX 5090 (Blackwell, sm_120)
• vLLM: 0.17.0
• Model: Qwen3.5-35B-A3B-AWQ, Qwen3.5-27B-AWQ
• CUDA: 12.8

Related Issues

• [Bug]: AssertionError in causal_conv1d_update when capturing CUDA graphs for Qwen3.5/GDN layers #35945 (same root cause, closed without merge)
• [Bug]: Qwen 3.5 27B AWQ 4bit capturing CUDA graph fails #35743 (Qwen3.5 AWQ CUDA graph failures)
• [Bugfix] Cap FULL decode cudagraph sizes for Mamba/hybrid models (#34094) #34571 (related fix for Mamba models, does not cover GDN path)

[gemini-code-assist]

Code Review

This pull request removes an incorrect assertion in causal_conv1d_update that causes an AssertionError when running Qwen3.5 models with Gated Delta Networks (GDN) layers. The assertion assert num_cache_lines >= batch is invalid when conv_state_indices is provided, as conv_state is a shared cache pool in that scenario, and num_cache_lines represents the cache size, not a per-batch dimension. The removal of this assertion is correct as the necessary batch validation is already handled by other checks.