per_device instead of per_gpu/error thrown when argument unknown
Modified the trainer argument so that `per_device_train_batch_size` and `per_device_eval_batch_size` are preferred over `per_gpu_*`.

`per_gpu_*` still works when `per_device_*` isn't used, but is deprecated.

The trainer argument parser now throws an error if an argument is unknown, only if the `return_remaining_strings` flag is kept to `False`. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4618?src=pr&el=h1) Report
> Merging [#4618](https://codecov.io/gh/huggingface/transformers/pull/4618?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/842588c12ffbbe3502c5ab4a18646ad31d9c1e34&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `50.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4618/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4618?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4618      +/-   ##
==========================================
- Coverage   78.02%   78.01%   -0.01%     
==========================================
  Files         124      124              
  Lines       20626    20635       +9     
==========================================
+ Hits        16093    16099       +6     
- Misses       4533     4536       +3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4618?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/hf\_argparser.py](https://codecov.io/gh/huggingface/transformers/pull/4618/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcmdwYXJzZXIucHk=) | `61.11% <0.00%> (-0.87%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4618/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.47% <0.00%> (ø)` | |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/4618/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `76.40% <58.33%> (-2.61%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4618/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4618/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4618?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/4618?src=pr&el=footer). Last update [842588c...eae844b](https://codecov.io/gh/huggingface/transformers/pull/4618?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
