add `is_decoder` attribute to `PretrainedConfig`

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`.

transformers/configuration_utils.py

@@ -56,6 +56,7 @@ class PretrainedConfig(object):
         self.torchscript = kwargs.pop('torchscript', False)
         self.use_bfloat16 = kwargs.pop('use_bfloat16', False)
         self.pruned_heads = kwargs.pop('pruned_heads', {})
+        self.is_decoder = kwargs.pop('is_decoder', False)
 
     def save_pretrained(self, save_directory):
         """ Save a configuration object to the directory `save_directory`, so that it