[Flax] Refactor gpt2 & bert example docs (#13024)
* fix_torch_device_generate_test
* remove @
* improve docs for clm
* speed-ups
* correct t5 example as well
* push final touches
* Update examples/flax/language-modeling/README.md
* correct docs for mlm
* Update examples/flax/language-modeling/README.md
Co-authored-by: Patrick von Platen <patrick@huggingface.co>