[testing] skip decorators: docs, tests, bugs
This PR:

* fixes a bug in `require_torch_and_cuda`
* makes all skip decorators consistent code-wise
* adds a test for testing combinations of skip decorators and other decorators
* clarifies `testing.rst` notes

OK, so other than a small bug in  `require_torch_and_cuda` our skip decorators can be used in any order.

The only problem I found so far is when they are used together with `@parameterized`, which has to come first and skip decorators last. It rewrites test names, to create a unique test name for each parameter group. and then it runs them - it has no idea it may have any skip decorators before it (The decorators get all stacked, and one below has no idea what the one above does).

If you find other unusual decorators, please let me know and I will investigate.

<!-- This line specifies which issue to close after the pull request is merged. -->
Partially fixes #7326

@LysandreJik, @sgugger 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=h1) Report
> Merging [#7334](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/25b0463d0ba3fcbcf7fff8aa4027a2d8e959364b?el=desc) will **decrease** coverage by `3.75%`.
> The diff coverage is `43.75%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7334/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7334      +/-   ##
==========================================
- Coverage   80.48%   76.73%   -3.76%     
==========================================
  Files         181      181              
  Lines       34827    34827              
==========================================
- Hits        28032    26724    -1308     
- Misses       6795     8103    +1308     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `67.28% <43.75%> (-1.24%)` | :arrow_down: |
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `21.62% <0.00%> (-78.38%)` | :arrow_down: |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `16.87% <0.00%> (-77.64%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `19.02% <0.00%> (-74.21%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `22.14% <0.00%> (-72.41%)` | :arrow_down: |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `60.00% <0.00%> (-30.00%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `71.21% <0.00%> (-12.88%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `81.81% <0.00%> (-4.55%)` | :arrow_down: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.93% <0.00%> (-0.34%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.64% <0.00%> (-0.28%)` | :arrow_down: |
| ... and [11 more](https://codecov.io/gh/huggingface/transformers/pull/7334/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=footer). Last update [25b0463...c17c310](https://codecov.io/gh/huggingface/transformers/pull/7334?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
