[Bugfix] Revert custom attention mask for gemma3-mm (#28995)

Signed-off-by: Isotr0py <mozf@mail2.sysu.edu.cn>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

vllm/transformers_utils/config.py

@@ -520,17 +520,6 @@ def is_interleaved(config: PretrainedConfig) -> bool:
     return False
 
 
-def uses_custom_attention_masks(config: PretrainedConfig) -> bool:
-    """Detect if model uses custom attention mask generation for multimodal.
-
-    Some multimodal models require custom attention masks that enable
-    bidirectional attention between image tokens while maintaining causal
-    attention for text tokens. Currently applies to Gemma3 multimodal models.
-    """
-    architectures = getattr(config, "architectures", [])
-    return "Gemma3ForConditionalGeneration" in architectures
-
-
 def _maybe_update_auto_config_kwargs(kwargs: dict[str, Any], model_type: str):
     """
     Update kwargs for AutoConfig initialization based on model_type