[CI][AMD]Bugfix] Check that model_config is not None in enable_norm_pad_fusion

[rasmith]

Purpose

If cfg.model_config is None, then vllm.enable_norm_pad_fusion will crash. This is causing the following failures in CI:

FAILED kernels/moe/test_routing_simulator.py::test_routing_strategy_integration - AttributeError: 'NoneType' object has no attribute 'get_hidden_size'
FAILED kernels/moe/test_shared_fused_moe_routed_transform.py::test_routed_input_transform_inside_vs_outside[dtype0-256-128-1] - AttributeError: 'NoneType' object has no attribute 'get_hidden_size'
FAILED kernels/moe/test_shared_fused_moe_routed_transform.py::test_routed_input_transform_inside_vs_outside[dtype0-256-128-32] - AttributeError: 'NoneType' object has no attribute 'get_hidden_size'
FAILED kernels/moe/test_shared_fused_moe_routed_transform.py::test_routed_input_transform_inside_vs_outside[dtype0-128-64-1] - AttributeError: 'NoneType' object has no attribute 'get_hidden_size'
FAILED kernels/moe/test_shared_fused_moe_routed_transform.py::test_routed_input_transform_inside_vs_outside[dtype0-128-64-32] - AttributeError: 'NoneType' object has no attribute 

Test Plan

pytest -sv kernels/moe

Test Result

1950 passed, 5399 skipped, 8 warnings

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request correctly addresses a potential AttributeError by adding a None check for cfg.model_config before accessing its attributes. This prevents a crash when model_config is not available. I have one suggestion to make the logic more robust by checking the model type directly instead of relying on a hardcoded hidden size, which will improve maintainability.

[yewentao256]

LGTM, thanks for the work!