Minor error on example distillation script
## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.5.1
- Platform: Linux-4.18.0-147.el8.x86_64-x86_64-with-Ubuntu-16.04-xenial
- Python version: 3.7.10
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: True
- Using distributed or parallel set-up in script?: True

### Who can help
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @Rocketknight1

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

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- for issues with a model report at https://discuss.huggingface.co/ and tag the model's creator.

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

 --> I think @VictorSanh might help since it's about a minor bug in distillation.

## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

examples/research_projects/distillation

The tasks I am working on is:
* [] an official GLUE/SQUaD task: (give the name)
* [X] my own task or dataset: (give details below)

It's not GLUE/SQUaD but official BookCorpus and Wikipedia datasets from `datasets`

## To reproduce

Steps to reproduce the behavior:

1. Convert concatenation of bookcorpus and Wikipedia text from `datasets` to `txt` file.
2. Separate it with `\n`
3. Run scripts following *A. Preparing the data*

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```
Traceback (most recent call last):
  File "train.py", line 322, in <module>
    main()
  File "train.py", line 223, in main
    init_gpu_params(args)
  File "/volume/compression_and_distillation/transformers/examples/distillation/utils.py", line 55, in init_gpu_params
    if params.n_gpu <= 0:
AttributeError: 'Namespace' object has no attribute 'n_gpu'
Traceback (most recent call last):
  File "train.py", line 322, in <module>
    main()
  File "train.py", line 223, in main
    init_gpu_params(args)
  File "/volume/compression_and_distillation/transformers/examples/distillation/utils.py", line 55, in init_gpu_params
    if params.n_gpu <= 0:
AttributeError: 'Namespace' object has no attribute 'n_gpu'
Traceback (most recent call last):
  File "train.py", line 322, in <module>
    main()
  File "train.py", line 223, in main
    init_gpu_params(args)
  File "/volume/compression_and_distillation/transformers/examples/distillation/utils.py", line 55, in init_gpu_params
    if params.n_gpu <= 0:
AttributeError: 'Namespace' object has no attribute 'n_gpu'
Traceback (most recent call last):
  File "train.py", line 322, in <module>
    main()
  File "train.py", line 223, in main
    init_gpu_params(args)
  File "/volume/compression_and_distillation/transformers/examples/distillation/utils.py", line 55, in init_gpu_params
    if params.n_gpu <= 0:
AttributeError: 'Namespace' object has no attribute 'n_gpu'
Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/venv/distill/lib/python3.7/site-packages/torch/distributed/launch.py", line 261, in <module>
    main()
  File "/home/venv/distill/lib/python3.7/site-packages/torch/distributed/launch.py", line 257, in main
    cmd=cmd)
```


## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

The error comes because of the inconsistency of variable name as `n_gpu` in the trainer.py script but `gpus` in parsing.

It can easily be solved changing `gpus` when parsing to `n_gpu`.