Fix F841 flake8 warning
This PR completes the "fix all flake8 warnings" effort of the last few days.

There's a lot of judgment in the fixes here: when the result of an expression is assigned to a variable that isn't used:

- if the expression has no side effect, then it can safely be removed
- if the expression has side effects, then it must be kept and only the assignment to a variable must be removed
- or it may be a coding / refactoring mistake that results in a badly named variable

I'm not sure I made the right call in all cases, so I would appreciate a review.

E203, E501, W503 are still ignored because they're debatable, black disagrees with flake8, and black wins (by not being configurable).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=h1) Report
> Merging [#2291](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/072750f4dc4f586cb53f0face4b4a448bb0cdcac?src=pr&el=desc) will **decrease** coverage by `1.18%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2291/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2291      +/-   ##
==========================================
- Coverage   74.45%   73.26%   -1.19%     
==========================================
  Files          85       85              
  Lines       14608    14603       -5     
==========================================
- Hits        10876    10699     -177     
- Misses       3732     3904     +172
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [...c/transformers/modeling\_tf\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `86.13% <ø> (+0.84%)` | :arrow_up: |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `78.86% <ø> (-0.18%)` | :arrow_down: |
| [src/transformers/data/metrics/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL21ldHJpY3MvX19pbml0X18ucHk=) | `27.9% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.28%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `81.05% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `96.54% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.12% <66.66%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `54.1% <0%> (-10.15%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `71.19% <0%> (-2.32%)` | :arrow_down: |
| ... and [5 more](https://codecov.io/gh/huggingface/transformers/pull/2291/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=footer). Last update [072750f...3e0cf49](https://codecov.io/gh/huggingface/transformers/pull/2291?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
