[Tokenization] Fix #5181 - make #5155 more explicit - move back the default logging level in tests to WARNING
Author: thomwolf
Pull request: yes

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

Comments:
# [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.