Fix/test convert_mbart.py
from [forums](https://discuss.huggingface.co/t/how-can-i-convert-a-model-created-with-fairseq/564/10?u=sshleifer)


```

    Traceback (most recent call last):
      File "./convert_mbart_original_checkpoint_to_pytorch.py", line 7, in <module>
        from .convert_bart_original_pytorch_checkpoint_to_pytorch import remove_ignore_keys_
    ModuleNotFoundError: No module named '__main__.convert_bart_original_pytorch_checkpoint_to_pytorch'; '__main__' is not a package
```

After I change `from .convert_bart_original_pytorch_checkpoint_to_pytorch import remove_ignore_keys_` to `from convert_bart_original_pytorch_checkpoint_to_pytorch import remove_ignore_keys_` (just removing the dot), the script can run
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