Fewer torch.hub requirements (#959)

Summary:
We will raise exceptions if these are needed and aren't available. Only keep minimum set of reqs
Pull Request resolved: https://github.com/pytorch/fairseq/pull/959

Differential Revision: D16623304

Pulled By: myleott

fbshipit-source-id: 8e65253742e393b527e8396a9433e64ebec9bb55

hubconf.py

@@ -11,12 +11,8 @@ from fairseq.models import MODEL_REGISTRY
 
 
 dependencies = [
-    'fastBPE',
     'regex',
     'requests',
-    'sacremoses',
-    'sentencepiece',
-    'subword_nmt',
     'torch',
 ]