New BartModel (#2745)

* Results same as fairseq
* Wrote a ton of tests
* Struggled with api signatures
* added some docs

src/transformers/modeling_encoder_decoder.py

@@ -236,42 +236,6 @@ class PreTrainedEncoderDecoder(nn.Module):
 
         return decoder_outputs + encoder_outputs
 
-    @staticmethod
-    def prepare_model_kwargs(**kwargs):
-        """ Prepare the encoder and decoder's keyword arguments.
-
-        Keyword arguments come in 3 flavors:
-        - encoder-specific (prefixed by `encoder_`)
-        - decoder-specific (prefixed by `decoder_`)
-        - those that apply to the model as whole.
-
-        We let the specific kwargs override the common ones in case of
-        conflict.
-        """
-        kwargs_common = {
-            argument: value
-            for argument, value in kwargs.items()
-            if not argument.startswith("encoder_") and not argument.startswith("decoder_")
-        }
-        decoder_kwargs = kwargs_common.copy()
-        encoder_kwargs = kwargs_common.copy()
-        encoder_kwargs.update(
-            {
-                argument[len("encoder_") :]: value
-                for argument, value in kwargs.items()
-                if argument.startswith("encoder_")
-            }
-        )
-        decoder_kwargs.update(
-            {
-                argument[len("decoder_") :]: value
-                for argument, value in kwargs.items()
-                if argument.startswith("decoder_")
-            }
-        )
-        decoder_kwargs["encoder_attention_mask"] = encoder_kwargs.get("attention_mask", None)
-        return encoder_kwargs, decoder_kwargs
-
 
 class Model2Model(PreTrainedEncoderDecoder):
     r"""