[PyTorch Bart] Split Bart into different models (#9343)

* first try

* remove old template

* finish bart

* finish mbart

* delete unnecessary line

* init pegasus

* save intermediate

* correct pegasus

* finish pegasus

* remove cookie cutter leftover

* add marian

* finish blenderbot

* replace in file

* correctly split blenderbot

* delete "old" folder

* correct "add statement"

* adapt config for tf comp

* correct configs for tf

* remove ipdb

* fix more stuff

* fix mbart

* push pegasus fix

* fix mbart

* more fixes

* fix research projects code

* finish docs for bart, mbart, and marian

* delete unnecessary file

* correct attn typo

* correct configs

* remove pegasus for seq class

* correct peg docs

* correct peg docs

* finish configs

* further improve docs

* add copied from statements to mbart

* fix copied from in mbart

* add copy statements to marian

* add copied from to marian

* add pegasus copied from

* finish pegasus

* finish copied from

* Apply suggestions from code review

* make style

* backward comp blenderbot

* apply lysandres and sylvains suggestions

* apply suggestions

* push last fixes

* fix docs

* fix tok tests

* fix imports code style

* fix doc

docs/source/model_doc/blenderbot.rst

@@ -43,13 +43,10 @@ Implementation Notes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Blenderbot uses a standard `seq2seq model transformer <https://arxiv.org/pdf/1706.03762.pdf>`__ based architecture.
-- It inherits completely from :class:`~transformers.BartForConditionalGeneration`
-- Even though blenderbot is one model, it uses two tokenizers :class:`~transformers.BlenderbotSmallTokenizer` for 90M
-  checkpoint and :class:`~transformers.BlenderbotTokenizer` for all other checkpoints.
-- :class:`~transformers.BlenderbotSmallTokenizer` will always return :class:`~transformers.BlenderbotSmallTokenizer`,
-  regardless of checkpoint. To use the 3B parameter checkpoint, you must call
-  :class:`~transformers.BlenderbotTokenizer` directly.
 - Available checkpoints can be found in the `model hub <https://huggingface.co/models?search=blenderbot>`__.
+- This is the `default` Blenderbot model class. However, some smaller checkpoints, such as
+  ``facebook/blenderbot_small_90M``, have a different architecture and consequently should be used with
+  `BlenderbotSmall <https://huggingface.co/transformers/master/model_doc/blenderbot_small.html>`__.
 
 
 Usage
@@ -59,26 +56,15 @@ Here is an example of model usage:
 
 .. code-block::
 
-        >>> from transformers import BlenderbotSmallTokenizer, BlenderbotForConditionalGeneration
-        >>> mname = 'facebook/blenderbot-90M'
+        >>> from transformers import BlenderbotTokenizer, BlenderbotForConditionalGeneration
+        >>> mname = 'facebook/blenderbot-400M-distill'
         >>> model = BlenderbotForConditionalGeneration.from_pretrained(mname)
-        >>> tokenizer = BlenderbotSmallTokenizer.from_pretrained(mname)
+        >>> tokenizer = BlenderbotTokenizer.from_pretrained(mname)
         >>> UTTERANCE = "My friends are cool but they eat too many carbs."
         >>> inputs = tokenizer([UTTERANCE], return_tensors='pt')
         >>> reply_ids = model.generate(**inputs)
-        >>> print([tokenizer.decode(g, skip_special_tokens=True, clean_up_tokenization_spaces=False) for g in reply_ids])
-
-
-Here is how you can check out config values:
-
-.. code-block::
-
-
-        >>> from transformers import BlenderbotConfig
-        >>> config_90 = BlenderbotConfig.from_pretrained("facebook/blenderbot-90M")
-        >>> config_90.to_diff_dict()  # show interesting Values.
-        >>> configuration_3B = BlenderbotConfig("facebook/blenderbot-3B")
-        >>> configuration_3B.to_diff_dict()
+        >>> print(tokenizer.batch_decode(reply_ids))
+        ["<s> That's unfortunate. Are they trying to lose weight or are they just trying to be healthier?</s>"]
 
 
 BlenderbotConfig
@@ -93,12 +79,6 @@ BlenderbotTokenizer
 .. autoclass:: transformers.BlenderbotTokenizer
     :members: build_inputs_with_special_tokens
 
-BlenderbotSmallTokenizer
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-.. autoclass:: transformers.BlenderbotSmallTokenizer
-    :members:
-
 
 BlenderbotModel
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -106,7 +86,7 @@ BlenderbotModel
 See :obj:`transformers.BartModel` for arguments to `forward` and `generate`
 
 .. autoclass:: transformers.BlenderbotModel
-    :members:
+    :members: forward
 
 
 BlenderbotForConditionalGeneration
@@ -115,7 +95,7 @@ BlenderbotForConditionalGeneration
 See :obj:`transformers.BartForConditionalGeneration` for arguments to `forward` and `generate`
 
 .. autoclass:: transformers.BlenderbotForConditionalGeneration
-    :members:
+    :members: forward
 
 
 TFBlenderbotForConditionalGeneration