adding TRANSFORMERS_VERBOSITY env var
Per discussion at https://github.com/huggingface/transformers/pull/6816#issuecomment-686347433, this PR:

- adds `TRANSFORMERS_VERBOSITY` env var
- docs
- tests
- new test utils

I'm open to a different name if that one doesn't work.

Thank you.


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=h1) Report
> Merging [#6961](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/56742e9f610231d7b28fe2387770dc56014b79de?el=desc) will **increase** coverage by `0.65%`.
> The diff coverage is `96.42%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6961/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6961      +/-   ##
==========================================
+ Coverage   80.00%   80.65%   +0.65%     
==========================================
  Files         161      161              
  Lines       30120    30147      +27     
==========================================
+ Hits        24097    24315     +218     
+ Misses       6023     5832     -191     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/testing\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90ZXN0aW5nX3V0aWxzLnB5) | `69.17% <94.11%> (+3.28%)` | :arrow_up: |
| [src/transformers/utils/logging.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy91dGlscy9sb2dnaW5nLnB5) | `85.89% <100.00%> (+10.89%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.29% <0.00%> (+0.32%)` | :arrow_up: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.71% <0.00%> (+0.80%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+1.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6961/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `98.95% <0.00%> (+73.82%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=footer). Last update [56742e9...3c194b7](https://codecov.io/gh/huggingface/transformers/pull/6961?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>All the requested changes have been done.<|||||>Great, thanks @stas00 