model: support output hidden states for DecoderOnly

[rebel-thkim]

Pull Request Description

⚠️ Important: Branch Target

• New features, enhancements, and non-critical fixes: Merge to dev branch
• Critical hotfixes only: Merge to main branch (must also merge to dev)

Please ensure you've selected the correct base branch before submitting!

Type of Change

• Release (dev → main merge for production release)
• New Model Support
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Other (please describe):

Changes Overview

Motivation and Context

Related Issues

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Conventional commit

type(optional scope): description

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Type candidate

• Model Updates
  • model: Adding New models or Bugfix for existing models
    • ex) Add LlavaNext
    • ex) Bugfix Whisper
• Enhancements
  • performance: Optimizing some models or this library itself
    • ex) Loading RBLNModel faster
    • ex) Optimizing Memory Usage of DecoderOnlyModel
• Code Refactor
  • refactor: Re-arrange class architecture, or more.
    • ex) Refactor Seq2Seq
• Documentation
  • doc: Update docstring only
• Library Dependencies
  • dependency: Update requirements, something like that.
• Release
  • release: Merging dev to main for production release
    • ex) Release v1.2.0
• Other
  • other: None of above.
    • ex) ci update
    • ex) pdm update

[Copilot]

Pull Request Overview

This PR adds support for outputting hidden states in DecoderOnly models. The implementation enables models to optionally return intermediate layer representations alongside their standard outputs.

Key changes:

• Added output_hidden_states parameter to model configurations and forward methods
• Implemented hidden states collection during both prefill and decode phases
• Updated output structures to include hidden states when requested

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/optimum/rbln/transformers/modeling_outputs.py	Added hidden_states field to RBLNDecoderOnlyOutput
src/optimum/rbln/transformers/models/decoderonly/configuration_decoderonly.py	Added output_hidden_states configuration parameter
src/optimum/rbln/transformers/models/decoderonly/decoderonly_architecture.py	Modified forward methods to collect and return hidden states from all layers
src/optimum/rbln/transformers/models/decoderonly/decoderonly_runtime_utils.py	Updated prefill and decode forward methods to handle hidden states output
src/optimum/rbln/transformers/models/decoderonly/modeling_decoderonly.py	Added hidden states collection logic for both prefill and decode phases
src/optimum/rbln/transformers/models/gemma3/modeling_gemma3.py	Extended Gemma3 model to support hidden states output

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[rebel-kblee]

can you add the test cases with rbln_config.output_hidden_states=True?

[rebel-jongho]

Can output_hidden_states in configuration_colqwen2.py be removed?

[rebel-thkim]

colqwen2 is refactored on the other PR @rebel-jongho