Added the ability to train with bf16 through arguments

[esslushy]

While fp16 causes error while training, I used bf16 for training and saw that it worked fine. I added the ability to enable this option through arguments and added an explanation to the tutorial. By default bf16 is off, but if you enable both the no_fp16 flag and bf16 flag, you train using mixed precision bf16.

[esslushy]

that was a good suggestion! I forgot not every platform supports that form of mixed precision! Let me know if anything else needs to be changed.

[loubnabnl]

LGTM thanks!