[trainer] implement support for full fp16 in evaluation/predict (#10268)
* implement --fp16_full_eval
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* style
* add test
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>