[FlaxBert] Add ForCausalLM (#16995)

* [FlaxBert] Add ForCausalLM

* make style

* fix output attentions

* Add RobertaForCausalLM

* remove comment

* fix fx-to-pt model loading

* remove comment

* add modeling tests

* add enc-dec model tests

* add big_bird

* add electra

* make style

* make repo-consitency

* add to docs

* remove roberta test

* quality

* amend cookiecutter

* fix attention_mask bug in flax bert model tester

* tighten pt-fx thresholds to 1e-5

* add 'copied from' statements

* amend 'copied from' statements

* amend 'copied from' statements

* quality

docs/source/en/model_doc/electra.mdx

@@ -158,6 +158,11 @@ This model was contributed by [lysandre](https://huggingface.co/lysandre). The o
 [[autodoc]] FlaxElectraForPreTraining
     - __call__
 
+## FlaxElectraForCausalLM
+
+[[autodoc]] FlaxElectraForCausalLM
+    - __call__
+
 ## FlaxElectraForMaskedLM
 
 [[autodoc]] FlaxElectraForMaskedLM