add base model classes to bart subclassed models

[patil-suraj]

What does this PR do?

This PR adds base model classes for MBart, Pegasus and Blenderbot, and adds these in MODEL_MAPPING dict.
This will enable to load these models using the AutoModel class and pipelines.

Right now these models can't be loaded using pipeline since pipeline relies on the AutoModel class.

transformers/src/transformers/pipelines.py

Lines 105 to 110 in a4b21cd

	if isinstance(model, str):
	if is_torch_available() and not is_tf_available():
	model = AutoModel.from_pretrained(model, revision=revision)
	elif is_tf_available() and not is_torch_available():
	model = TFAutoModel.from_pretrained(model, revision=revision)
	else:

[patrickvonplaten]

Looks good, only thing left to do is to add those new models to the docs as well. To pegasus.rst, ... respectively

[sgugger]

LGTM, thanks for adding those!