Make default_data_collator more flexible and deprecate old behavior
This PR does two things:

- avoid breaking changes when people had a custom `DataCollator` with a `collate_batch` method (there is still the breaking change with `DataCollator` not being a class anymore)
- makes `default_data_collator` more flexible by handling dicts on top of `InputExamples` classes coming from our examples

This fixes #5049
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=h1) Report
> Merging [#5060](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d5477baf7d87b9bdad386f2f317732b85277b06b&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `84.21%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5060/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5060      +/-   ##
==========================================
+ Coverage   77.41%   77.43%   +0.01%     
==========================================
  Files         130      130              
  Lines       22023    22029       +6     
==========================================
+ Hits        17050    17059       +9     
+ Misses       4973     4970       -3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5060/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <50.00%> (+0.09%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/5060/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `98.33% <93.33%> (+8.67%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=footer). Last update [d5477ba...857975c](https://codecov.io/gh/huggingface/transformers/pull/5060?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Refactored on @julien-c suggestion and removed the little hack to handle tensors and lists of ints (which was cauding a 4x slowdown on my tests). I just use the first features to test if we have a tensor (then stack) or not (then use torch.tensor).<|||||>LGTM!