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The introduction of a decoder introduces 2 changes:
- We need to be able to specify a separate mask in the cross
attention to mask the positions corresponding to padding tokens in the
encoder state.
- The self-attention in the decoder needs to be causal on top of not
attending to padding tokens.

the definition of `get_masks` would blow with the proper combination of
arguments. It was just a matter of moving a definition outside of a
control structure.

We currenctly instantiate encoders and decoders for the seq2seq by
passing the `is_decoder` keyword argument to the `from_pretrained`
classmethod. On the other hand, the model class looks for the value
of the `is_decoder` attribute in its config.

In order for the value to propagate from the kwarg to the configuration
we simply need to define `is_decoder` as an attribute to the base
`PretrainedConfig`, with a default at `False`.