Add missing support for Flax XLM-RoBERTa (#15900)

* Adding Flax XLM-RoBERTa

* Add Flax to __init__

* Adding doc and dummy objects

* Add tests

* Add Flax XLM-R models autodoc

* Fix tests

* Add Flask XLM-RoBERTa to TEST_FILES_WITH_NO_COMMON_TESTS

* Update src/transformers/models/xlm_roberta/modeling_flax_xlm_roberta.py

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

* Update tests/xlm_roberta/test_modeling_flax_xlm_roberta.py

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

* Update tests/xlm_roberta/test_modeling_flax_xlm_roberta.py

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

* Remove test on large Flask XLM-RoBERTa

* Add tokenizer to the test

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

docs/source/model_doc/xlm-roberta.mdx

@@ -124,3 +124,33 @@ This model was contributed by [stefan-it](https://huggingface.co/stefan-it). The
 
 [[autodoc]] TFXLMRobertaForQuestionAnswering
     - call
+
+## FlaxXLMRobertaModel
+
+[[autodoc]] FlaxXLMRobertaModel
+    - __call__
+
+## FlaxXLMRobertaForMaskedLM
+
+[[autodoc]] FlaxXLMRobertaForMaskedLM
+    - __call__
+
+## FlaxXLMRobertaForSequenceClassification
+
+[[autodoc]] FlaxXLMRobertaForSequenceClassification
+    - __call__
+
+## FlaxXLMRobertaForMultipleChoice
+
+[[autodoc]] FlaxXLMRobertaForMultipleChoice
+    - __call__
+
+## FlaxXLMRobertaForTokenClassification
+
+[[autodoc]] FlaxXLMRobertaForTokenClassification
+    - __call__
+
+## FlaxXLMRobertaForQuestionAnswering
+
+[[autodoc]] FlaxXLMRobertaForQuestionAnswering
+    - __call__