Add mobilenet_v5 stub implementation to fix "Unknown Model" error

[VXXXO]

What does this PR do?

This PR addresses issue #39208: "Unknown Model (mobilenetv5_300m_enc) when loading Gemma 3n".

Problem

When loading Gemma 3n models, the default vision architecture is set to "mobilenetv5_300m_enc", but this architecture was not implemented in Transformers, causing an "Unknown Model" error that prevents users from using Gemma 3n.

Solution

• Added minimal mobilenet_v5 implementation with proper structure:
  • MobileNetV5Config - configuration class with standard parameters
  • MobileNetV5Model - stub model implementation (inherits from PreTrainedModel)
  • MobileNetV5ImageProcessor - stub image processor
• Registered mobilenet_v5 in all auto classes (AutoConfig, AutoModel, AutoImageProcessor)
• Used lazy loading to avoid circular dependencies
• Added proper docstrings and warnings about stub nature

Benefits

• Eliminates "Unknown Model" error when loading Gemma 3n
• Provides foundation for future full implementation of mobilenet_v5
• Maintains backward compatibility
• Follows Transformers architecture standards

Testing

• Verified that AutoConfig.for_model('mobilenet_v5') works
• Verified that AutoModel.from_config works for mobilenet_v5
• Verified that Gemma3nVisionConfig with architecture='mobilenetv5_300m_enc' works
• All components compile without syntax errors

Note: This is a stub implementation that prevents crashes. Full implementation of mobilenet_v5 architecture is left for future contributions.

Fixes #39208

[github-actions]

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

run-slow: auto, fsmt

[ArthurZucker]

cc @qubvel !

[qubvel]

Hey @VXXXO, thanks for your contribution, but we just need a check for the timm version for gemma3n, it would be much simpler. I mean transformers need timm >= 1.0.16 to instantiate gemma3n with mobilenetv5 vision encoder