[Tokenization] Fix #5181 - make #5155 more explicit - move back the default logging level in tests to WARNING
Padding to max sequence length while truncating to another length did not behave as expected on slow tokenizers as raised in #5181 by @sshleifer (it was truncating and then padding back to original length...)

This PR adds more tests to cover various combinations of padding + truncation strategies.

Fix #5181 

This PR also:
- make #5155 clearer by changing the assertion in a cleaner error message (until the data processors are refactored)
- move back the default level of logging in tests to `logging.WARNING`
- switch some really slow tokenizations tests on CPU (when the inputs go in the full models) to `@slow` and speed-up testing by limiting the max sequence length used for testing
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=h1) Report
> Merging [#5252](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7ac91107119f95a9034e5404bd5af34355d0ffa5&el=desc) will **increase** coverage by `1.60%`.
> The diff coverage is `86.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5252/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5252      +/-   ##
==========================================
+ Coverage   77.48%   79.09%   +1.60%     
==========================================
  Files         138      138              
  Lines       24073    24071       -2     
==========================================
+ Hits        18653    19038     +385     
+ Misses       5420     5033     -387     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `91.48% <85.71%> (-1.00%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.16% <100.00%> (+0.34%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.35% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+1.41%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `78.65% <0.00%> (+2.29%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `99.14% <0.00%> (+2.56%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.62% <0.00%> (+3.53%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `91.41% <0.00%> (+42.11%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/huggingface/transformers/pull/5252/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=footer). Last update [7ac9110...230551f](https://codecov.io/gh/huggingface/transformers/pull/5252?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok, this one is ready for review/merge.
It fixes an important bug in the interaction of padding and truncation for slow tokenizers in the new backend.
It also adds a lot of tests and tweaks a bit the tests to make them faster and less verbose.