Clean up diffs in Trainer/TFTrainer
This PR does a bit of cleanup in the two Trainer and tries ti make the diff in the two TrainingArguments as minimal as possible.

- `set_seed` is now just one function in trainer_utils: the problem was that even if you only use TF and import it from transformers right now, it does not set seed for tf **and** will fail on PyTorch stuff.
- `eval_steps` is now a common argument for both versions of Trainer
- as discussed, `n_gpu` in `TFTrainingArguments` becomes `n_replicas`. This is a breaking change, I can add the deprecation warnings that goes with it if you think it's necessary.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=h1) Report
> Merging [#5417](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/64e3d966b1131c15b5905b1e1e582d4bebac1ef0&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `65.11%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5417/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5417   +/-   ##
=======================================
  Coverage   77.75%   77.75%           
=======================================
  Files         140      140           
  Lines       24373    24392   +19     
=======================================
+ Hits        18951    18967   +16     
- Misses       5422     5425    +3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `16.53% <33.33%> (-0.45%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.96% <40.00%> (-0.85%)` | :arrow_down: |
| [src/transformers/training\_args\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzX3RmLnB5) | `47.45% <44.44%> (-3.71%)` | :arrow_down: |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.22% <100.00%> (ø)` | |
| [src/transformers/trainer\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3V0aWxzLnB5) | `85.71% <100.00%> (+7.45%)` | :arrow_up: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `77.55% <100.00%> (+0.46%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `66.25% <0.00%> (-32.52%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.26% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `96.82% <0.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.18% <0.00%> (+0.50%)` | :arrow_up: |
| ... and [4 more](https://codecov.io/gh/huggingface/transformers/pull/5417/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=footer). Last update [64e3d96...c185e2f](https://codecov.io/gh/huggingface/transformers/pull/5417?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Pretty cool 🔥