Summary:
- `FairseqModel` -> `FairseqEncoderDecoderModel`
- add `FairseqDecoder.extract_features` and `FairseqDecoder.output_layer`
- `encoder_out_dict` -> `encoder_out`
- rm unused `remove_head` functions
- update docs
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/561

Differential Revision: D15271142

Pulled By: myleott

fbshipit-source-id: 8e8864e399336020f0271c780598e968ff51a264