Fix Trainer in DataParallel setting
The new output types seem to break data parallel FYI, see comment on #5671. This is is because of the line 
```
return type(out)(map(gather_map, zip(*outputs)))
```
in `scatter_gather` which tries to reconstruct an output of the same type as ours (and fails since it does not provide the necessary arguments). There is no  way to fix our `ModelOutput` to work with this AFAICT.

However, we have the `return_tuple` argument to fix the issue :-)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=h1) Report
> Merging [#5685](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7fad617dc1fc681a7f5da5e0172c8b83f4bf0024&el=desc) will **decrease** coverage by `0.20%`.
> The diff coverage is `25.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5685/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5685      +/-   ##
==========================================
- Coverage   78.11%   77.91%   -0.21%     
==========================================
  Files         146      146              
  Lines       25983    25987       +4     
==========================================
- Hits        20297    20247      -50     
- Misses       5686     5740      +54     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.84% <25.00%> (-0.12%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `44.56% <0.00%> (-46.35%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/5685/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/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `79.94% <0.00%> (-6.02%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.92% <0.00%> (-1.97%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.31% <0.00%> (+1.28%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5685/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.18% <0.00%> (+74.91%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=footer). Last update [7fad617...05ec8f6](https://codecov.io/gh/huggingface/transformers/pull/5685?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
