examples/text-classification/run_pl.sh multiple problems
Fixing this sequence of errors - each fix required for the next error

running:
```
cd examples/text-classification
./run_pl.sh
```

error 1:
```
Traceback (most recent call last):
  File "run_pl_glue.py", line 183, in <module>
    trainer = generic_train(model, args)
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 289, in generic_train
    if args.gpus > 1:
AttributeError: 'Namespace' object has no attribute 'gpus'
```
solution: added `--n_gpus` arg

error 2:
```
Traceback (most recent call last):
  File "run_pl_glue.py", line 183, in <module>
    trainer = generic_train(model, args)
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 300, in generic_train
    **train_params,
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 853, in from_argparse_args
    return cls(**trainer_kwargs)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 468, in __init__
    self.tpu_cores = _parse_tpu_cores(tpu_cores)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/distrib_parts.py", line 526, in _parse_tpu_cores
    raise MisconfigurationException("`tpu_cores` can only be 1, 8 or [<1-8>]")
pytorch_lightning.utilities.exceptions.MisconfigurationException: `tpu_cores` can only be 1, 8 or [<1-8>]
```
solution: removed `default=0` for `tpu_cores`

error 3:
```
Traceback (most recent call last):
  File "run_pl_glue.py", line 183, in <module>
    trainer = generic_train(model, args)
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 304, in generic_train
    trainer.fit(model)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 1038, in fit
    model.setup('fit')
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 125, in setup
    dataloader = self.get_dataloader("train", train_batch_size)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/torch/nn/modules/module.py", line 594, in __getattr__
    type(self).__name__, name))
AttributeError: 'GLUETransformer' object has no attribute 'get_dataloader'
```
solution: added a wrapper - but it's incomplete - what to do with the `shuffle` arg?

error 4:
```
Traceback (most recent call last):
  File "run_pl_glue.py", line 187, in <module>
    trainer = generic_train(model, args)
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 306, in generic_train
    trainer.fit(model)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 1044, in fit
    results = self.run_pretrain_routine(model)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 1213, in run_pretrain_routine
    self.train()
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 370, in train
    self.run_training_epoch()
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 452, in run_training_epoch
    batch_output = self.run_training_batch(batch, batch_idx)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 632, in run_training_batch
    self.hiddens
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 776, in optimizer_closure
    hiddens)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 956, in training_forward
    output = self.model.training_step(*args)
  File "run_pl_glue.py", line 44, in training_step
    tensorboard_logs = {"loss": loss, "rate": self.lr_scheduler.get_last_lr()[-1]}
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/torch/nn/modules/module.py", line 594, in __getattr__
    type(self).__name__, name))
AttributeError: 'GLUETransformer' object has no attribute 'lr_scheduler'
```
solution: I'm not sure how it used to work, but there is no `self.lr_scheduler` in pytorch-lightning (PL), I found one here: `self.trainer.lr_schedulers[0]["scheduler"]` and set this attribute. I have no idea whether this always works. Someone who wrote this script would probably know better where the missing attribute has gone. It's set inside `def fit`/CPU but inside the `trainer` object and not `nn.Module`.

Further notes:

`run_pl.sh` invokes PL in CPU mode, despite available GPU. I haven't tested this on gpu yet - I just saw during debug that PL [inits optimizers](https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/trainer/trainer.py#L1096) just before it runs `run_pretrain_routine`, so I didn't find an easy PL predefined method where one could preset `self.lr_scheduler`. 

Perhaps PL API has changed and caused this issue?

error 5:
```
Traceback (most recent call last):
  File "run_pl_glue.py", line 218, in <module>
    trainer = generic_train(model, args)
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 305, in generic_train
    trainer.fit(model)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 1044, in fit
    results = self.run_pretrain_routine(model)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 1213, in run_pretrain_routine
    self.train()
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 370, in train
    self.run_training_epoch()
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 452, in run_training_epoch
    batch_output = self.run_training_batch(batch, batch_idx)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/training_loop.py", line 671, in run_training_batch
    self.on_batch_end()
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/trainer/callback_hook.py", line 82, in on_batch_end
    callback.on_batch_end(self, self.get_model())
  File "/mnt/nvme1/code/huggingface/transformers-issue-1/examples/lightning_base.py", line 198, in on_batch_end
    lrs = {f"lr_group_{i}": lr for i, lr in enumerate(self.lr_scheduler.get_lr())}
AttributeError: 'LoggingCallback' object has no attribute 'lr_scheduler'
```
solution: see notes for error 4.

with these fixes the code at least starts training, I didn't test further, since clearly there is a better way. Only the fixes for the first 2 errors are obviously correct to merge.

All the fixes are in one PR, as one can't move to the next error, before fixing the previous ones.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=h1) Report
> Merging [#6028](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c69ea5efc4eac65b183e8d07b1bf91d20bbe0c8c&el=desc) will **increase** coverage by `0.18%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6028/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6028      +/-   ##
==========================================
+ Coverage   78.50%   78.68%   +0.18%     
==========================================
  Files         146      146              
  Lines       26249    26249              
==========================================
+ Hits        20606    20655      +49     
+ Misses       5643     5594      -49     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6028/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6028/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (+0.75%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6028/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <0.00%> (+69.51%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=footer). Last update [c69ea5e...414fbe9](https://codecov.io/gh/huggingface/transformers/pull/6028?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Looks like both of us worked on this at the same time - a different solution - https://github.com/huggingface/transformers/pull/6027<|||||>The code of this PR eventually got in the hard way via 3 PRs: https://github.com/huggingface/transformers/pull/6027, https://github.com/huggingface/transformers/pull/6307 and https://github.com/huggingface/transformers/pull/6314