feat: Add GPT-OSS support via mcore

[ashors1]

What does this PR do ?

NOTE: blocked on vllm-project/vllm#27722

Issues

closes #869

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you run the unit tests and functional tests locally? Visit our Testing Guide for how to run tests
• Did you add or update any necessary documentation? Visit our Document Development Guide for how to write, build and test the docs.

Additional Information

• ...

[github-actions]

❌ Submodule Fast-Forward Check Failed

Check based on commit: 9c9711a (PR #1452 from ashors/gpt-oss-tot)

✅ Submodules that are properly updated:

Megatron-LM: ✅ PR branch is ahead of main branch (fast-forward)

❌ Submodules that need attention:

Megatron-Bridge: ❌ Commits have DIVERGED from a common ancestor
TARGET (main branch): https://github.com/NVIDIA-NeMo/Megatron-Bridge/commits/62f4704b8d665ac4a8c318a809a070217caa8901/
CURRENT (PR #1452 from ashors/gpt-oss-tot): https://github.com/NVIDIA-NeMo/Megatron-Bridge/commits/7367429371012ca6aa4f78f56091b9dbb325c12d/

Please ensure all submodule commits are fast-forwards of the main branch before merging.

[github-actions]

✅ Submodule Fast-Forward Check Results

Check based on commit: 0f7e8e8 (PR #1452 from ashors/gpt-oss-tot)

✅ Submodules that are properly updated:

Megatron-Bridge: ✅ PR branch is ahead of main branch (fast-forward)
Megatron-LM: ✅ PR branch is ahead of main branch (fast-forward)

All submodule changes look good! ✨