Have documentation fail on warning
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5189?src=pr&el=h1) Report
> Merging [#5189](https://codecov.io/gh/huggingface/transformers/pull/5189?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1262495a912b9cd97e2ae174fd627a9d8a502341&el=desc) will **increase** coverage by `36.63%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5189/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5189?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master    #5189       +/-   ##
===========================================
+ Coverage   41.35%   77.99%   +36.63%     
===========================================
  Files         138      138               
  Lines       23772    23772               
===========================================
+ Hits         9831    18540     +8709     
+ Misses      13941     5232     -8709     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5189?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (+0.63%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `91.07% <0.00%> (+0.71%)` | :arrow_up: |
| [src/transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvdXRpbHMucHk=) | `27.63% <0.00%> (+1.31%)` | :arrow_up: |
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.37% <0.00%> (+1.44%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `76.42% <0.00%> (+2.66%)` | :arrow_up: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `94.52% <0.00%> (+2.73%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.81% <0.00%> (+3.54%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3V0aWxzLnB5) | `69.77% <0.00%> (+5.54%)` | :arrow_up: |
| [src/transformers/configuration\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3Q1LnB5) | `96.42% <0.00%> (+10.71%)` | :arrow_up: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `76.84% <0.00%> (+12.63%)` | :arrow_up: |
| ... and [47 more](https://codecov.io/gh/huggingface/transformers/pull/5189/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5189?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/5189?src=pr&el=footer). Last update [1262495...47a9e15](https://codecov.io/gh/huggingface/transformers/pull/5189?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>The `build_doc` test now fails when there's a warning, e.g. on the second commit:

```
reading sources... [ 85%] multilingual
reading sources... [ 87%] notebooks
reading sources... [ 89%] pretrained_models
reading sources... [ 91%] quickstart
reading sources... [ 93%] serialization
reading sources... [ 95%] summary
reading sources... [ 97%] torchscript
reading sources... [100%] usage


Warning, treated as error:
/home/circleci/transformers/src/transformers/modeling_albert.py:docstring of transformers.AlbertModel.forward:47:Unexpected indentation.
make: *** [Makefile:19: html] Error 2

Exited with code exit status 2
CircleCI received exit code 2
```