MBartForConditionalGeneration (#6441)

* add MBartForConditionalGeneration

* style

* rebase and fixes

* add mbart test in TEST_FILES_WITH_NO_COMMON_TESTS

* fix docs

* don't ignore mbart

* doc

* fix mbart fairseq link

* put mbart before bart

* apply doc suggestions

utils/check_repo.py

@@ -30,6 +30,7 @@ TEST_FILES_WITH_NO_COMMON_TESTS = [
     "test_modeling_tf_xlm_roberta.py",
     "test_modeling_xlm_roberta.py",
     "test_modeling_pegasus.py",
+    "test_modeling_mbart.py",
 ]
 
 # Update this list for models that are not documented with a comment explaining the reason it should not be.
@@ -106,7 +107,6 @@ def get_model_test_files():
         "test_modeling_common",
         "test_modeling_encoder_decoder",
         "test_modeling_marian",
-        "test_modeling_mbart",
         "test_modeling_tf_common",
     ]
     test_files = []