loading pre-trained checkpoint failed

@eric_hammy I set AGENT = TorchBeastAgent in submission_config.py, and MODEL_DIR = "./saved_models/torchbeast/pretrained_0.5B" in agents/torchbeast_agent.py. Then I ran $ python test_submission.py but it gives me the following error message. (commit id: 3f9ef7f1)

Traceback (most recent call last):
  File "test_submission.py", line 36, in <module>
    evaluate()
  File "test_submission.py", line 25, in evaluate
    agent = Agent(num_envs, batched_env.num_actions)
  File "/data/private/research/AgentLearning/nethack_challenge/agents/torchbeast_agent.py", line 26, in __init__
    self.model = load_model(MODEL_DIR, self.device)
  File "/data/private/research/AgentLearning/nethack_challenge/nethack_baselines/torchbeast/models/__init__.py", line 54, in load_model
    checkpoint_states = torch.load(flags.checkpoint, map_location=device)
  File "/root/anaconda3/envs/nle_challenge/lib/python3.8/site-packages/torch/serialization.py", line 607, in load
    return _load(opened_zipfile, map_location, pickle_module, **pickle_load_args)
  File "/root/anaconda3/envs/nle_challenge/lib/python3.8/site-packages/torch/serialization.py", line 882, in _load
    result = unpickler.load()
  File "/root/anaconda3/envs/nle_challenge/lib/python3.8/site-packages/omegaconf/basecontainer.py", line 108, in __setstate__
    key_type = d["_metadata"].key_type
KeyError: '_metadata'