fix FSDP ShardedGradScaler
# What does this PR do?

1. Fixes #18350 by renaming the import for FSDP ShardedGradScaler so that it doesn't change the scope of globally imported Fairscale ShardedGradScaler
2. Another minor change of raising error when transformer auto wrap class name is not found in the model.

## Before submitting
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- [ ] Did you write any new necessary tests?

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