[trainer + examples] set log level from CLI (#12276)
* set log level from CLI
* add log_level_replica + test + extended docs
* cleanup
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* rename datasets objects to allow datasets module
* improve the doc
* style
* doc improve
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
