Empty assert hunt
Author: TevenLeScao
Pull request: yes

Empty asserts are bad for debugging. I tried to remove them all and to add helpful Pytorch-style messages with the shapes of the corresponding objects when they were mismatched-lengths checks + the file paths when they were file-not-found checks.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=h1) Report
> Merging [#6056](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/12f14710cea72a2104ff698762a8fc68a5dc0a0b&el=desc) will **decrease** coverage by `0.35%`.
> The diff coverage is `22.91%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6056/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6056      +/-   ##
==========================================
- Coverage   78.82%   78.47%   -0.36%     
==========================================
  Files         146      146              
  Lines       26200    26204       +4     
==========================================
- Hits        20653    20564      -89     
- Misses       5547     5640      +93     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/commands/train.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy90cmFpbi5weQ==) | `0.00% <ø> (ø)` | |
| [src/transformers/data/metrics/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL21ldHJpY3MvX19pbml0X18ucHk=) | `26.66% <0.00%> (ø)` | |
| [src/transformers/data/metrics/squad\_metrics.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL21ldHJpY3Mvc3F1YWRfbWV0cmljcy5weQ==) | `0.00% <0.00%> (ø)` | |
| [src/transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvdXRpbHMucHk=) | `27.63% <0.00%> (ø)` | |
| [src/transformers/data/processors/xnli.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMveG5saS5weQ==) | `27.08% <0.00%> (-2.47%)` | :arrow_down: |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `82.04% <0.00%> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.39% <0.00%> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `81.55% <0.00%> (ø)` | |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `85.88% <0.00%> (ø)` | |
| [src/transformers/modeling\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tb2JpbGViZXJ0LnB5) | `89.45% <0.00%> (ø)` | |
| ... and [17 more](https://codecov.io/gh/huggingface/transformers/pull/6056/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=footer). Last update [12f1471...09c7880](https://codecov.io/gh/huggingface/transformers/pull/6056?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I'm in love 😍 😍. Thanks @TevenLeScao !

@LysandreJik can I merge this?