text: Title: TypeError: cannot unpack non-iterable NoneType object
  Body: ## 🐛 Bug
  I met this bug when I try to load a fairseq translation model. I could see the same issue is still open in fairseq GitHub repo. I tried installing torch and torchvision packages as mentioned in the below link but still I am facing the same issue.
  [](https://github.com/facebookresearch/fairseq/issues/4214)
  
  
  ### To Reproduce
  Using your  [colab tutorial](https://colab.research.google.com/github/pytorch/pytorch.github.io/blob/master/assets/hub/pytorch_fairseq_translation.ipynb) will also produce the same bug.
  
  ```
  Using cache found in /root/.cache/torch/hub/pytorch_fairseq_main
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  <ipython-input-21-61d0ed709261> in <module>
        1 # Load translation model
        2 # en2ru = torch.hub.load('pytorch/fairseq', 'transformer.wmt19.en-ru.single_model', tokenizer='moses', bpe='fastbpe')
  ----> 3 ru2en = torch.hub.load('pytorch/fairseq', 'transformer.wmt19.ru-en.single_model', tokenizer='moses', bpe='fastbpe')
  
  7 frames
  /usr/local/lib/python3.7/dist-packages/torch/hub.py in load(repo_or_dir, model, source, trust_repo, force_reload, verbose, skip_validation, *args, **kwargs)
  
  /usr/local/lib/python3.7/dist-packages/torch/hub.py in _load_local(hubconf_dir, model, *args, **kwargs)
  
  /usr/local/lib/python3.7/dist-packages/torch/hub.py in _import_module(name, path)
       87     return '[https://github.com/{}/{}/archive/{}.zip](https://github.com/%7B%7D/%7B%7D/archive/%7B%7D.zip)'.format(repo_owner, repo_name, branch)
       88 
  ---> 89 
       90 def _load_attr_from_module(module, func_name):
       91     # Check if callable is defined in the module
  
  /usr/lib/python3.7/importlib/_bootstrap_external.py in exec_module(self, module)
  
  /usr/lib/python3.7/importlib/_bootstrap.py in _call_with_frames_removed(f, *args, **kwds)
  
  ~/.cache/torch/hub/pytorch_fairseq_main/hubconf.py in <module>
       37 
       38 # only do fairseq imports after checking for dependencies
  ---> 39 from fairseq.hub_utils import (  # noqa; noqa
       40     BPEHubInterface as bpe,
       41     TokenizerHubInterface as tokenizer,
  
  ~/.cache/torch/hub/pytorch_fairseq_main/fairseq/__init__.py in <module>
       31 hydra_init()
       32 
  ---> 33 import fairseq.criterions  # noqa
       34 import fairseq.distributed  # noqa
       35 import fairseq.models  # noqa
  
  ~/.cache/torch/hub/pytorch_fairseq_main/fairseq/criterions/__init__.py in <module>
       22     CRITERION_DATACLASS_REGISTRY,
       23 ) = registry.setup_registry(
  ---> 24     "--criterion", base_class=FairseqCriterion, default="cross_entropy"
       25 )
       26 
  
  TypeError: cannot unpack non-iterable NoneType object
  ```
  
  
  
  #### Code sample
  Using your  [colab tutorial](https://colab.research.google.com/github/pytorch/pytorch.github.io/blob/master/assets/hub/pytorch_fairseq_translation.ipynb) will also produce the same bug.
  
  ### Expected behavior
  
  successfully load the translation model
  
  ### Environment
  
   - PyTorch Version (e.g., 1.6)
   - Google colab
  