Update lang-pairs path and add fixed-dictionary for small models (#3084)
Update lang-pairs path and add fixed-dictionary for small models (#3084)

Summary:
With missing file extension in --lang-pairs option generation from 418M and 1.2B Models fails with the following error
```
ValueError: language pair en-fr contains languages that are not in the language dictionary; langs: ['language_pairs_small_models']
```

However generation still fails after restoring file extension with following error:
```
RuntimeError: Error(s) in loading state_dict for TransformerModel:
	size mismatch for encoder.embed_tokens.weight: copying a param with shape torch.Size([128112, 1024]) from checkpoint, the shape in current model is torch.Size([128104, 1024]).
	size mismatch for decoder.embed_tokens.weight: copying a param with shape torch.Size([128112, 1024]) from checkpoint, the shape in current model is torch.Size([128104, 1024]).
	size mismatch for decoder.output_projection.weight: copying a param with shape torch.Size([128112, 1024]) from checkpoint, the shape in current model is torch.Size([128104, 1024]).
```
This could be resolved by adding --fixed-dictionary model_dict.128k.txt like in Generation for the 12B model section.

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3084

Reviewed By: huihuifan

Differential Revision: D26225960

Pulled By: myleott

fbshipit-source-id: 0cabe1fd074e45484264d551117704180c7ade9f

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq