[trainer] conditional ctx managers into one wrapper (#14663)
* [trainer] conditional ctx managers into one wrapper
* workaround for contextlib.nullcontext for py<3.7
* Update src/transformers/trainer.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* one more autocast
* style
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>