DataParallel fixes
1. switched to a more precise check as suggested by @thomwolf

```
-        if self.args.n_gpu > 1:
+        if isinstance(model, nn.DataParallel):
```
  discussion: https://github.com/huggingface/transformers/issues/5693#issuecomment-657937349

2. fix tests - require the same fixup under DataParallel as the training module fix merged earlier today:
https://github.com/huggingface/transformers/pull/5685
discussion: https://github.com/huggingface/transformers/issues/5693#issuecomment-657938856


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5733?src=pr&el=h1) Report
> Merging [#5733](https://codecov.io/gh/huggingface/transformers/pull/5733?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c3c61ea01733403210a1d159114e8c3d042dabb7&el=desc) will **increase** coverage by `1.34%`.
> The diff coverage is `50.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5733/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5733?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5733      +/-   ##
==========================================
+ Coverage   77.22%   78.57%   +1.34%     
==========================================
  Files         146      146              
  Lines       26012    26012              
==========================================
+ Hits        20088    20439     +351     
+ Misses       5924     5573     -351     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5733?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.84% <50.00%> (ø)` | |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `63.55% <0.00%> (-31.78%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.71% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5733/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5733?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/5733?src=pr&el=footer). Last update [c3c61ea...4d61a1e](https://codecov.io/gh/huggingface/transformers/pull/5733?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Way better fix, LGTM 

Did the multigpu test pass?<|||||>> Way better fix, LGTM

Well, I followed @sgugger and @thomwolf's breadcrumbs, so that was easy.

> Did the multigpu test pass?

Yes. And I retested the glue_run.py on multi-gpu machine.
<|||||>Pushed another fix for https://github.com/huggingface/transformers/issues/5693#issuecomment-659564678