Fix FP16 and attention masks in FunnelTransformer
Author: sgugger
Pull request: yes

This `.float()` should have been removed, it was necessary before I converted the attention masks to floating types at the beginning of the forward of the Encoder, but it's now useless (and bad for mixed precision as shown in #7371).

Also, the attentions masks were used the wrong way (0 for non-masked tokens, 1 for masked) which was incompatible with the way transformers tokenizers work.

Fixes #7371 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7374?src=pr&el=h1) Report
> Merging [#7374](https://codecov.io/gh/huggingface/transformers/pull/7374?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0ccb6f5c6da9e703766e8053581fddfc6dcc71a9?el=desc) will **decrease** coverage by `1.43%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7374/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7374?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7374      +/-   ##
==========================================
- Coverage   78.20%   76.76%   -1.44%     
==========================================
  Files         181      181              
  Lines       35751    35750       -1     
==========================================
- Hits        27959    27444     -515     
- Misses       7792     8306     +514     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7374?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19mdW5uZWwucHk=) | `86.72% <100.00%> (-0.02%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mdW5uZWwucHk=) | `94.04% <100.00%> (ø)` | |
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `21.62% <0.00%> (-78.38%)` | :arrow_down: |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `16.71% <0.00%> (-77.89%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `21.53% <0.00%> (-68.15%)` | :arrow_down: |
| [src/transformers/tokenization\_pegasus.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcGVnYXN1cy5weQ==) | `46.03% <0.00%> (-49.21%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `18.73% <0.00%> (-39.79%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `74.14% <0.00%> (-18.70%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.36% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `83.37% <0.00%> (+0.24%)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/huggingface/transformers/pull/7374/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7374?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/7374?src=pr&el=footer). Last update [0ccb6f5...705ee7a](https://codecov.io/gh/huggingface/transformers/pull/7374?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Yes let's keep it open until the problem is fully solved.

@LysandreJik this is ready for review and to be merged. Confirmed I can overfit the training set on a sequence classification task and train with the `fp16` flag so this should solve all problems with FunnelTransformer.

There's a failing Funnel integration that should be taken care of before merging.