Add handling for Qwen3VLMoe on older transformers versions

[fynnsu]

SUMMARY:
#1981 added Qwen3VLMoe with associated tests, however this model isn't available on all transformers versions that we support. Therefore, (similar to #2030) this pr ensures we don't import or test the model when using a transformers version that doesn't support it.

TEST PLAN:
Confirmed that this change fixes import llmcompressor when using oldest support transformers version 4.54.0. Ran test with old transformers version (test gets skipped) and new transformers version (test passes).

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[gemini-code-assist]

Summary of Changes

Hello @fynnsu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses compatibility issues with the Qwen3VLMoe model across different versions of the transformers library. It ensures that the llmcompressor library can gracefully handle environments where this specific model is not available, preventing import errors and allowing relevant tests to be skipped appropriately. This change improves the robustness and backward compatibility of the system by adapting to varying transformers library capabilities.

Highlights

• Conditional Import Handling: Implemented conditional imports for Qwen3VLMoe related classes within src/llmcompressor/modeling/qwen3_vl_moe.py by utilizing TYPE_CHECKING to prevent runtime import errors on older transformers versions.
• Test Skipping for Compatibility: Added a pytest.mark.skipif decorator to test_calib_qwen3_vl_moe_module in tests/llmcompressor/modeling/test_calib_qwen3_vl_moe.py. This ensures the test is skipped if the Qwen3VLMoe model is not available in the installed transformers version, maintaining compatibility.
• Type Hint Refinement: Updated type hints in src/llmcompressor/modeling/qwen3_vl_moe.py to directly reference Qwen3VLMoeTextSparseMoeBlock after moving the transformers imports into a TYPE_CHECKING block, simplifying the code.

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[gemini-code-assist]

Code Review

This pull request aims to provide compatibility with older versions of the transformers library by making the import of Qwen3VLMoe components conditional. The changes in the test file are well-implemented, using a try-except block to gracefully handle missing dependencies and skip tests. However, the corresponding change in the source file src/llmcompressor/modeling/qwen3_vl_moe.py is less robust. I've provided a suggestion to improve its reliability by adopting a similar try-except pattern, which will prevent potential runtime errors.