Trainer multi label
This is a follow-up from #7126. The same kinds of models that can output multiple predictions expect multiple labels (not named "labels") so the evaluation code needs to be changed for this. To support models built by users, I added a `label_names` field in the `TrainingArguments` which contain the label names. It then defaults to `["labels"]` for most models, `["start_positions", "end_positions"]` for question answering models if the user does not set it to work seamlessly for all Transformers models.

I ended up writing a few util functions that concat/numpify for tensors or nested lists/tuples of tensors to avoid testing everywhere in `Trainer`, I think the design is cleaner this way and it also supports model with crazy outputs (if we set `output_attentions=True` for instance). I also added a test for the multiple labels predictions.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=h1) Report
> Merging [#7191](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3babef815c5372d039202b2247a4ed6afb2a410a?el=desc) will **decrease** coverage by `1.45%`.
> The diff coverage is `70.83%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7191/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7191      +/-   ##
==========================================
- Coverage   80.86%   79.41%   -1.46%     
==========================================
  Files         169      169              
  Lines       32293    32322      +29     
==========================================
- Hits        26115    25668     -447     
- Misses       6178     6654     +476     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3V0aWxzLnB5) | `58.88% <56.52%> (-1.41%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `55.72% <80.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `91.74% <100.00%> (+0.07%)` | :arrow_up: |
| [...c/transformers/modeling\_tf\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `10.00% <0.00%> (-76.00%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `20.85% <0.00%> (-71.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsLnB5) | `19.85% <0.00%> (-68.29%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `71.59% <0.00%> (-23.38%)` | :arrow_down: |
| [src/transformers/modeling\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19seG1lcnQucHk=) | `70.01% <0.00%> (-20.75%)` | :arrow_down: |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `79.21% <0.00%> (-10.25%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `72.25% <0.00%> (-10.00%)` | :arrow_down: |
| ... and [13 more](https://codecov.io/gh/huggingface/transformers/pull/7191/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=footer). Last update [3babef8...29ce623](https://codecov.io/gh/huggingface/transformers/pull/7191?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
