[Flax] Align FlaxBertForMaskedLM with BertForMaskedLM, implement from_pretrained, init (#9054)

* save intermediate

* save intermediate

* save intermediate

* correct flax bert model file

* new module / model naming

* make style

* almost finish BERT

* finish roberta

* make fix-copies

* delete keys file

* last refactor

* fixes in run_mlm_flax.py

* remove pooled from run_mlm_flax.py`

* fix gelu | gelu_new

* remove Module from inits

* splits

* dirty print

* preventing warmup_steps == 0

* smaller splits

* make fix-copies

* dirty print

* dirty print

* initial_evaluation argument

* declaration order fix

* proper model initialization/loading

* proper initialization

* run_mlm_flax improvements: improper model inputs bugfix + automatic dataset splitting + tokenizers parallelism warning + avoiding warmup_steps=0 bug

* removed tokenizers warning hack, fixed model re-initialization

* reverted training_args.py changes

* fix flax from pretrained

* improve test in flax

* apply sylvains tips

* update init

* make 0.3.0 compatible

* revert tevens changes

* revert tevens changes 2

* finalize revert

* fix bug

* add docs

* add pretrained to init

* Update src/transformers/modeling_flax_utils.py

* fix copies

* final improvements

Co-authored-by: TevenLeScao <teven.lescao@gmail.com>

src/transformers/file_utils.py

@@ -270,6 +270,7 @@ TRANSFORMERS_CACHE = os.getenv("TRANSFORMERS_CACHE", PYTORCH_TRANSFORMERS_CACHE)
 WEIGHTS_NAME = "pytorch_model.bin"
 TF2_WEIGHTS_NAME = "tf_model.h5"
 TF_WEIGHTS_NAME = "model.ckpt"
+FLAX_WEIGHTS_NAME = "flax_model.msgpack"
 CONFIG_NAME = "config.json"
 MODEL_CARD_NAME = "modelcard.json"