Change model_kwargs argument to encoder_outputs to support transformers>=4.22.1

[SamSamhuns]

Newer versions of the transformer library do a _validate_model_kwargs call inside transformers/generation_utils.py that check if the model arguments match between the decoder forward and _prepare_input_ids_for_generation function calls.

[gwkrsrch]

Hi @SamSamhuns , deeply thanks for this PR! Is this change compatible with transformers<4.22.1 ?

[SamSamhuns]

Tested with transformers==4.22.0 from pip install "transformers>=4.11.3,<4.22.1", it works for me