DOC: add BFloat16 dtype and BFloat16Tensor (#37051)
Summary:
Related to gh-36318
Mention `bfloat16` dtype and `BFloat16Tensor` in documentation. The real fix would be to implement cpu operations on 16-bit float `half`, and I couldn't help but notice that `torch.finfo(torch.bfloat16).xxx` crashes for `xxx in ['max', 'min', 'eps']`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37051
Differential Revision: D21476851
Pulled By: ngimel
fbshipit-source-id: fef601d3116d130d67cd3a5654077f31b699409b
