[FIX] ShardedGradScaler should only be set for FP16

[SeanNaren]

What does this PR do?

Closes #12914

When running with sharded training + bf16, I noticed that the ShardedGradScaler was being passed. This is incorrect as BF16 does not require any scaling logic. Fixed this and added a unit test!

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cc @Borda @SeanNaren @awaelchli @rohitgr7 @akihironitta @carmocca @justusschock