gpt-oss DTensor SFT support

[terrykong]

https://openai.com/index/introducing-gpt-oss/

[qiaochuz-nv]

QA RCCA Analysis

RCCA Status: Done
RCCA Category: covered
Test Action: none

Fix PR: #1470
Root Cause: fix PR includes upstream test coverage.
Trigger Config: gpt-oss DTensor SFT support

Coverage Assessment:

Area	Result
Existing Test	Upstream fix PR test coverage
Coverage Status	COVERED
Gap	N/A
Corrective Action	No nmfw_tests addition required; fix PR includes upstream test coverage or equivalent regression coverage.

Conclusion: RCCA classification completed for this closed NeMo-RL GitHub issue.