[doc] bf16/tf32 guide (#14579)
* [doc] bf16/tf32 guide
* expand
* expand
* Update docs/source/performance.md
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>