Migrate RegNet to standardized output tracing

[karthiksuki]

What does this PR do?

This PR migrates the RegNet model to the standardized output collection interface as part of the ongoing refactoring effort in issue #43979.

Specifically:

• Adds the _can_record_outputs dictionary to RegNetPreTrainedModel.
• Adds the @capture_outputs decorator to the base RegNetModel forward pass.
• Adds the @can_return_tuple decorator to the RegNetForImageClassification forward pass.
• Removes manual handling of output_attentions, output_hidden_states, and return_dict logic, delegating this to the decorators.
• Simplifies the code by removing manual loops for collecting hidden states.

Ref #43979

Who can review?

@molbap (Reviewer - tracking issue #43979)
@yonigozlan (Vision models)

[github-actions]

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

run-slow: regnet