Remove redundant hidden states
The quickstart showcasing the usage of the Model2Model currently fails. This is due to a positional argument that should be a named argument.

As I understand it, the `encoder_hidden_states` are already present in the `kwargs_decoder` dictionary, there is therefore no need to pass it to the decoder forward call.

With the current quickstart example this crashes as the position of the `encoder_hidden_states` means it's passed as an `attention_mask`.

Please correct me if I'm wrong @rlouf @thomwolf 