add custom datasets tutorial
A tutorial showing examples for working with custom datasets on several tasks. Goals:

1. Keep it general. The point is to show people how to use their own datasets, so don't use any processors or utilities that are dataset-specific.
2. Show several tasks with different data formats. I include sequence classification with IMDb, token classification with W-NUT NER, and question answering with squad 2.0. Also link to how to train a language model blog post.
3. Prepare the data in a way that works with Trainer, TFTrainer, native PyTorch, and native TensorFlow with keras's `fit` method.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=h1) Report
> Merging [#6466](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6e0b1dc8954b87c18f77a82000e81e02683b8eb1&el=desc) will **decrease** coverage by `1.34%`.
> The diff coverage is `83.53%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6466/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6466      +/-   ##
==========================================
- Coverage   79.77%   78.42%   -1.35%     
==========================================
  Files         148      153       +5     
  Lines       27214    28001     +787     
==========================================
+ Hits        21710    21960     +250     
- Misses       5504     6041     +537     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/data/test\_generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Rlc3RfZ2VuZXJhdGlvbl91dGlscy5weQ==) | `0.00% <0.00%> (ø)` | |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `90.00% <ø> (-0.91%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.15% <ø> (-0.20%)` | :arrow_down: |
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `12.25% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `25.63% <4.00%> (-54.16%)` | :arrow_down: |
| [src/transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb24ucHk=) | `25.55% <7.14%> (-70.00%)` | :arrow_down: |
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `51.92% <28.57%> (-20.81%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `37.84% <37.50%> (-0.18%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.38% <50.00%> (+1.79%)` | :arrow_up: |
| ... and [72 more](https://codecov.io/gh/huggingface/transformers/pull/6466/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=footer). Last update [7bc0056...31ea640](https://codecov.io/gh/huggingface/transformers/pull/6466?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Don't mind the failing test, it's been fixed on `master`.