Fix/timm version check gemma3n
Author: sameerajashyam
Pull request: yes

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Fixes # (issue)


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Adds a version check in `Gemma3nForConditionalGeneration` to ensure `timm >= 0.9.16`.

This prevents users from accidentally using outdated `timm` versions that don’t support models like `mobilenetv5_300m_enc`, and raises a clear `ImportError` if the requirement isn’t met.
Fixes #39208



Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gemma3n