* Implement head_mask for Flax BERT and other models copied from BERT
* Remove `from jax._src.nn.functions import sigmoid`
Remove `from jax._src.nn.functions import sigmoid` unintentionally added by IDE
* Remove no more valid copy statement
* Apply patil-suraj's suggestions from code review
* Apply suggestions from the code review
* Update Flax template
* Fix a typo
* Also update template for CausalLM modules
* add electra model to flax
* Remove Electra Next Sentence Prediction model added by mistake
* fix parameter sharing and loosen equality threshold
* fix styling issues
* add mistaken removen imports
* fix electra table
* Add FlaxElectra to automodels and fixe docs
* fix issues pointed out the PR
* fix flax electra to comply with latest changes
* remove stale class
* add copied from
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
