docs(pretrained_models): fix num parameters
@julien-c  This PR corrects the number of parameters of BERT based models.

Sometimes the difference between a given model and its pairs is important. For instance:
`bert-base-uncased` has **110M parameters** but `bert-base-multilingual-cased` has more than **178M parameters**, even if both models share the same architecture (12-layer, 768-hidden, 12-heads).

The difference is due to the vocabulary size:
`bert-base-uncased` uses a vocabulary of **30k** entries while `bert-base-multilingual-cased` uses a vocabulary of **119k** entries.

To compute the number of parameters:
``` python
from transformers import AutoModelForMaskedLM

bert_base = AutoModelForMaskedLM.from_pretrained('bert-base-uncased')
print(bert_base.num_parameters())

bert_multiling = AutoModelForMaskedLM.from_pretrained('bert-base-multilingual-cased')
print(bert_multiling.num_parameters())
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6043?src=pr&el=h1) Report
> Merging [#6043](https://codecov.io/gh/huggingface/transformers/pull/6043?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7f65daa2e155ecdd8594e19862dac8b322ed3b73&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6043/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6043?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6043      +/-   ##
==========================================
+ Coverage   79.57%   79.63%   +0.06%     
==========================================
  Files         146      146              
  Lines       26597    26597              
==========================================
+ Hits        21164    21181      +17     
+ Misses       5433     5416      -17     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6043?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6043/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `84.52% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6043/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `90.09% <0.00%> (+23.42%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6043?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/6043?src=pr&el=footer). Last update [7f65daa...ee22cce](https://codecov.io/gh/huggingface/transformers/pull/6043?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>@LysandreJik I created a new Pull Request (https://github.com/huggingface/transformers/pull/7575) to resolve the above mentioned conflicts and start from a dedicated branch.

Thanks.
