[CI] Fix MHA attention test failure (AttributeError when model_config is None in ViT attention backend)

[LucasWilkinson]

Fixes AttributeError: 'NoneType' object has no attribute 'multimodal_config' in get_vit_attn_backend() and is_vit_use_data_parallel() functions.

Breaking Commit: 9ad7f89f5 - [Models]: Make Multimodal config implicit in ViT implementation (#31972)

FIX #33028

Root Cause

The breaking commit introduced new functions get_vit_attn_backend() and is_vit_use_data_parallel() in vllm/model_executor/models/vision.py that access vllm_config.model_config.multimodal_config. However, the code only catches AssertionError from get_current_vllm_config(), but doesn't handle the case where model_config itself is None.

When tests use the default_vllm_config fixture, get_current_vllm_config() returns a VllmConfig where model_config is None. Accessing .multimodal_config on None raises AttributeError, which isn't caught.

Fix

Added a null check for model_config before accessing multimodal_config:

Test

python -m pytest tests/kernels/attention/test_mha_attn.py::test_mha_attn_varlen_forward[cuda-dtype2-64-1-16-var_seq_len0] -v

[gemini-code-assist]

Code Review

This pull request effectively addresses an AttributeError that occurred when model_config was None within get_vit_attn_backend() and is_vit_use_data_parallel() functions. The implemented null checks for model_config before accessing multimodal_config correctly resolve the issue, preventing potential crashes and improving the robustness of the system.

[Copilot]

Pull request overview

This PR fixes an AttributeError that occurs when model_config is None in ViT attention backend functions, particularly affecting tests that use the default_vllm_config fixture.

Changes:

• Added null checks for model_config before accessing multimodal_config in get_vit_attn_backend() and is_vit_use_data_parallel() functions
• Prevents AttributeError: 'NoneType' object has no attribute 'multimodal_config' when tests use VllmConfig() without a model_config

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