fix(vlm): use FP32 master weights for Nemotron Omni (#3444)

* fix(vlm): use FP32 master weights for Nemotron Omni

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* chore(vlm): remove unused embedding freeze setting

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

* refactor(vlm): drop projector freezing changes from Nemotron Omni fix

Remove the projector-freezing feature and the freeze_embeddings config
change, keeping only the FP32 master-weights optimizer and activation
checkpointing changes.

- restore freeze_config to the original tower-wide freezing (freeze_embeddings
  reinstated, freeze_vision_projector/freeze_audio_projector removed)
- revert apply_parameter_freezing to its main-branch behavior
- drop the projector-freezing unit tests and freeze_config recipe assertions

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

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Signed-off-by: HuiyingLi <willwin.lee@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>