Add mlm pretraining xla torch readme (#12011)
* fix_torch_device_generate_test
* remove @
* upload
* Apply suggestions from code review
* Apply suggestions from code review
* Apply suggestions from code review
* Update examples/flax/language-modeling/README.md
* add more info
* finish
* fix
Co-authored-by: Patrick von Platen <patrick@huggingface.co>