add base model classes to  bart subclassed models
# 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.
https://github.com/huggingface/transformers/blob/a4b21cdd20328f71448123ce7c962a78a5d75612/src/transformers/pipelines.py#L105-L110

