GitLab

* trainer and finetune_trainer enhancements and fixes

* add fallback default

* move the fixing of incorrect keys back into finetune trainer

* s/eval/val/ to match the split

* trainer can now use a different prefix than eval_ for metrics

* document new arg

* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* use 'eval' as the default for metric_key_prefix

* complete adjust var names + disambiguate

* fix logger

* add clarifying comment

* add clarifying comment

* style

* Apply suggestions from code review
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/trainer.py
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* complete removal of optional for metric_key_prefix

* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>