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abc86c0e - Enable BF16 autocast to everything during FP8 + some tweaks to enable FSDP (#2655)

Enable BF16 autocast to everything during FP8 + some tweaks to enable FSDP (#2655) * Basic autocasting stuff * Delay fp8 autocast until after DDP wrapping * More fixes * Bookmark: without dtype change * Bookmark: with dtype changes * Different alternative, better results * Didn't matter what order, same result * Revert + maintain * Fin * Refactor based on feedback * native_amp bool * Final nits