[trainer] add tf32-mode control (#14606)
* [trainer] add --tf32 support
* it's pt>=.17
* it's pt>=.17
* flip the default to True
* add experimental note
* simplify logic
* style
* switch to 3-state logic
* doc
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* re-style code
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>