Remove outdated BERT tips
Author: JetRunner
Pull request: yes

Why remove the tips:

> - BERT is a model with absolute position embeddings so it's usually advised to pad the inputs on
>   the right rather than the left.

Yes but since we don't provide an option to pad from the left I think it's not necessary.

> - BERT was trained with a masked language modeling (MLM) objective. It is therefore efficient at predicting masked
>   tokens and at NLU in general, but is not optimal for text generation. Models trained with a causal language
>   modeling (CLM) objective are better in that regard.

No. T5 & BART proved it wrong.

> - Alongside MLM, BERT was trained using a next sentence prediction (NSP) objective using the [CLS] token as a sequence
>   approximate. The user may use this token (the first token in a sequence built with special tokens) to get a sequence
>   prediction rather than a token prediction. However, averaging over the sequence may yield better results than using
>   the [CLS] token.

No. [CLS] can do learnable self-attention pooling, which is way much better than parameter-free average pooling especially when fine-tuned. (w.r.t. SentenceBERT)


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=h1) Report
> Merging [#6217](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/06f1692b023a701ab2bb443fa4f0bdd58c6bd234&el=desc) will **decrease** coverage by `0.64%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6217/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6217      +/-   ##
==========================================
- Coverage   79.53%   78.88%   -0.65%     
==========================================
  Files         146      146              
  Lines       26586    26586              
==========================================
- Hits        21145    20973     -172     
- Misses       5441     5613     +172     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_outputs.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vdXRwdXRzLnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `16.26% <0.00%> (-66.67%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `68.14% <0.00%> (-25.67%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `80.30% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/tokenization\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZHByLnB5) | `57.65% <0.00%> (+4.50%)` | :arrow_up: |
| [src/transformers/tokenization\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/6217/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fY3RybC5weQ==) | `96.11% <0.00%> (+17.47%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=footer). Last update [b6b2f22...8a7c591](https://codecov.io/gh/huggingface/transformers/pull/6217?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I'd personally leave the first two tricks (users may have their custom script for padding and BERT is not good at language generation). For the third, as discussed, I agree.
The docstrings of `BaseModelOutputWithPooling` should be changed accordingly, but I can take care of that in a separate PR.

@sgugger I've restored tips no.1 and updated tips no.2. Also took care of `BaseModelOutputWithPooling`.