Running "convert_graph_to_onnx.py" doesn't work.
## Environment info
- `transformers` version: 4.5.0.dev0
- Platform: Darwin-19.6.0-x86_64-i386-64bit
- Python version: 3.7.2
- PyTorch version (GPU?): 1.7.1 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@patrickvonplaten, @patil-suraj

Models:
Bart (https://huggingface.co/transformers/model_doc/marian.html)

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## Information

A problem arises when I run "python3 convert_graph_to_onnx.py" where I receive the following error message
```
Traceback (most recent call last):
  File "convert_graph_to_onnx.py", line 22, in <module>
    from .file_utils import ModelOutput, is_tf_available, is_torch_available
ModuleNotFoundError: No module named '__main__.file_utils'; '__main__' is not a package
```

## To reproduce
Run "python3 convert_graph_to_onnx.py" inside the following directory transformers/src/transformers.

Steps to reproduce the behavior:

1. cd "transformers/src/transformers/"
2. python3 convert_graph_to_onnx.py

## Expected behavior

I expect convert_graph_to_onnx.py to begin running.
This appears to be the proper way to run it.

`python3 -m transformers.convert_graph_to_onnx --framework pt --model bert-base-cased bert-base-cased.onnx`