fallback to float16 if bfloat16 is not supported

[GuichardVictor]

Parts of the code still used bfloat16 even when "use_bfloat" was False.
Float16 could be a replacement in case bfloat16 is not supported but still requested.

If the user asked for bfloat16 but device does not support it, make the autocast dtype be float16.

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