Fix backward compatibility issue with MFSDP --grad-reduce-in-bf16

[shjwudp]

What does this PR do ?

In the Megatron-FSDP code path, the new flag --megatron-fsdp-grad-comm-dtype overrides the previous --grad-reduce-in-bf16 functionality. This patch restores the --grad-reduce-in-bf16 behavior for backward compatibility.

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[shjwudp]

/ok to test 15c2369

[shjwudp]

/claude review

[shjwudp]

/ok to test a89014b