ci: add a job that checks if submodules are fast forwarded

[terrykong]

It's easy to overlook submodule changes so this introduces a job that checks if the submodule update can be fast-forwarded to. If it can't it could mean that the feature branch didn't incorporate commits on main.

Since it's possible someone would rebase, non-fast-forwarded shouldn't gate a PR, so this change just makes a comment on the PR to investigate to see if it's intentional

[github-actions]

❌ Submodule Fast-Forward Check Failed

Check based on commit: 2f5a368 (PR #695 from tk-ff-submodule-check)

❌ Submodules that need attention:

NeMo: ❌ Commits have DIVERGED from a common ancestor
TARGET (main branch): https://github.com/NVIDIA/NeMo/commits/33259f2540af6eef375d43fc48bdcbd7ec490c29/
CURRENT (PR #695 from tk-ff-submodule-check): https://github.com/NVIDIA/NeMo/commits/30e6820dbd47cf46371c41a2e40cb2e9cef51f6b/

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

[github-actions]

❌ Submodule Fast-Forward Check Failed

Check based on commit: 2fdf229 (PR #695 from tk-ff-submodule-check)

❌ Submodules that need attention:

NeMo: ❌ Commits have DIVERGED from a common ancestor
TARGET (main branch): https://github.com/NVIDIA/NeMo/commits/33259f2540af6eef375d43fc48bdcbd7ec490c29/
CURRENT (PR #695 from tk-ff-submodule-check): https://github.com/NVIDIA/NeMo/commits/30e6820dbd47cf46371c41a2e40cb2e9cef51f6b/

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