Fix mixed precision fine-tuning for text-to-image-lora-sdxl example. (#6751)
* Fix mixed precision fine-tuning for text-to-image-lora-sdxl example.
* fix text_encoder_two bug.
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Co-authored-by: Sajad Norouzi <sajadn@dev-dsk-sajadn-2a-87239470.us-west-2.amazon.com>