perf: update sft and dpo recipes to use bf16

[ashors1]

What does this PR do ?

SFT FP32 vs BF16:

DPO FP32 vs BF16:

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[terrykong]

@ashors1 could you share the convergence curves just for posterity

[ashors1]

@ashors1 could you share the convergence curves just for posterity

Updated the PR description

[terrykong]

@ashors1 there was a test failure https://github.com/NVIDIA/NeMo-RL/actions/runs/14841614037/job/41665718613

the functional sft test fails b/c the numerics have changed

[ashors1]

the functional sft test fails b/c the numerics have changed

Yep, looking into it now

[ashors1]

@terrykong updated the test and manually verified that the increased loss is not a concern, as over time the curves are comparable