Flax t5 Encoder (#17784)

* first draft adding Flax-t5-encoder and Flax-mt5-encoder

* imports

* after make fixup

* flax t5 encoder test

* black on test

* make fix-copies

* clean

* all_model_classes -> tuple

* clean test

* is_encoder_decoder=False in t5-enc tester

* remove file docstring before FlaxT5Encoder

* black

* isort

* commit suggestions on src/transformers/models/t5/modeling_flax_t5.py

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* commit suggestions on src/transformers/models/t5/modeling_flax_t5.py

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* Apply suggestions from code review

Co-authored-by: Suraj Patil <surajp815@gmail.com>

* remove _get_encoder_module

* self.decoder_seq_length -> self.encoder_seq_length as t5-enc does not have decoder

* bugfix - self.module_class is class itself, not instance;

* docs for mt5 and t5

* call -> __call__ in t5 doc

* FlaxMT5EncoderModel to TYPE_HINT

* run doc-builder to allow change the files

Co-authored-by: Suraj Patil <surajp815@gmail.com>

docs/source/en/model_doc/mt5.mdx

@@ -96,3 +96,7 @@ See [`T5TokenizerFast`] for all details.
 ## FlaxMT5ForConditionalGeneration
 
 [[autodoc]] FlaxMT5ForConditionalGeneration
+
+## FlaxMT5EncoderModel
+
+[[autodoc]] FlaxMT5EncoderModel

docs/source/en/model_doc/t5.mdx

@@ -371,3 +371,8 @@ T5 is supported by several example scripts, both for pre-training and fine-tunin
     - __call__
     - encode
     - decode
+
+## FlaxT5EncoderModel
+
+[[autodoc]] FlaxT5EncoderModel
+    - __call__