Doc pipelines
Author: sgugger
Pull request: yes

Continue the improvement of the main classes documentation with pipelines.

[Preview](https://67155-155220641-gh.circle-artifacts.com/0/docs/_build/html/main_classes/pipelines.html) of the new pipeline page.
[Preview](https://67155-155220641-gh.circle-artifacts.com/0/docs/_build/html/internal/pipelines_utils.html) of the new pipeline utils page.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6175?src=pr&el=h1) Report
> Merging [#6175](https://codecov.io/gh/huggingface/transformers/pull/6175?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d951c14ae46ee36b76981588ed6d03ab353ad766&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6175/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6175?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #6175   +/-   ##
=======================================
  Coverage   79.51%   79.52%           
=======================================
  Files         146      146           
  Lines       26607    26618   +11     
=======================================
+ Hits        21156    21167   +11     
  Misses       5451     5451           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6175?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/6175/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `79.79% <100.00%> (+0.42%)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6175/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `84.52% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6175/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `92.71% <0.00%> (-1.13%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6175/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `80.05% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/6175/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.77% <0.00%> (+23.94%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6175?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/6175?src=pr&el=footer). Last update [d951c14...1ec7448](https://codecov.io/gh/huggingface/transformers/pull/6175?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).