Example: PyTorch Lightning returns missing attribute error (Token Classification)
Hi,

the following error message is currently thrown (version 3.0.0 of Transformers) when running the `run_ner_pl.sh` example from the token classification example:

```bash
Traceback (most recent call last):
  File "run_pl_ner.py", line 198, in <module>
    trainer = generic_train(model, args)
  File "/mnt/transformers-stefan/examples/lightning_base.py", line 330, in generic_train
    trainer.fit(model)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/trainer.py", line 918, in fit
    self.single_gpu_train(model)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/distrib_parts.py", line 176, in single_gpu_train
    self.run_pretrain_routine(model)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/trainer.py", line 1093, in run_pretrain_routine
    self.train()
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/training_loop.py", line 335, in train
    self.reset_train_dataloader(model)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/data_loading.py", line 189, in reset_train_dataloader
    self.train_dataloader = self.request_dataloader(model.train_dataloader)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/trainer/data_loading.py", line 352, in request_dataloader
    dataloader = dataloader_fx()
  File "/mnt/transformers-stefan/examples/lightning_base.py", line 142, in train_dataloader
    * float(self.hparams.num_train_epochs)
  File "/usr/local/lib/python3.6/dist-packages/pytorch_lightning/utilities/parsing.py", line 116, in __getattr__
    raise AttributeError(f'Missing attribute "{key}"')
AttributeError: Missing attribute "n_gpu"
```

It seems that `n_gpu` is set in the `lightning_base.py` file:

https://github.com/huggingface/transformers/blob/7f60e93ac5c73e74b5a00d57126d156be9dbd2b8/examples/lightning_base.py#L139-L142

The example is running when `n_gpu` is changed to `gpus`.

I will prepare for fixing it and hopefully this does not introduce regression bugs for other PL examples 😅
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.
