error while instantiating model
Hi,

For my experiments, I need to make some changes to the forward pass of Roberta sequence classification model. Thus, I copied **RobertaForSequenceClassification** from _modeling_roberta.py_ into a [separate file](https://github.com/kevinghst/mixmatch_from_scratch/blob/master/models_roberta.py) in my repo.

And I try to instantiate it in my _main.py_ like this:
```
    model = RobertaForSequenceClassification.from_pretrained(
        'roberta-base',
        num_labels = NUM_LABELS[cfg.task],
    )
```
However, it gives me the following stacktrace:
----------------------------------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/torch/serialization.py", line 186, in _check_seekable
    f.seek(f.tell())
AttributeError: 'NoneType' object has no attribute 'seek'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/transformers/modeling_utils.py", line 516, in from_pretrained
    state_dict = torch.load(resolved_archive_file, map_location="cpu")
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/torch/serialization.py", line 368, in load
    return _load(f, map_location, pickle_module)
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/torch/serialization.py", line 517, in _load
    _check_seekable(f)
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/torch/serialization.py", line 189, in _check_seekable
    raise_err_msg(["seek", "tell"], e)
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/torch/serialization.py", line 182, in raise_err_msg
    raise type(e)(msg)
AttributeError: 'NoneType' object has no attribute 'seek'. You can only torch.load from a file that is seekable. Please pre-load the data into a buffer like io.BytesIO and try to load from it instead.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "main.py", line 208, in <module>
    num_labels = NUM_LABELS[cfg.task],
  File "/scratch/wz1232/anaconda3/envs/mixmatch/lib/python3.6/site-packages/transformers/modeling_utils.py", line 519, in from_pretrained
    "Unable to load weights from pytorch checkpoint file. "
OSError: Unable to load weights from pytorch checkpoint file. If you tried to load a PyTorch model from a TF 2.0 checkpoint, please set from_tf=True.
-----------------------------------------------------------------------------------------------------------

It doesn't occur when I import **RobertaForSequenceClassification** straight from the library, as opposed to from my own file containing the copied code. Why is this?
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
