modified method simple_accuracy(), solve the exception
Author: 7will10
Pull request: yes

modified method simple_accuracy(),
before:
it's (preds == labels).mean()
This will cause an exception[AttributeError: 'bool' object has no attribute 'mean'],
then after update:
change to accuracy_score(labels,preds),
use this method accuracy_score() in package sklearn.metrics

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=h1) Report
> Merging [#2545](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7833dfccac0d7d74e12d2b2be1f6caa6e895ca73?src=pr&el=desc) will **increase** coverage by `29.34%`.
> The diff coverage is `89.55%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2545/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #2545       +/-   ##
==========================================
+ Coverage   45.25%   74.6%   +29.34%     
==========================================
  Files          87      87               
  Lines       14800   14802        +2     
==========================================
+ Hits         6698   11043     +4345     
+ Misses       8102    3759     -4343
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bert\_japanese.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydF9qYXBhbmVzZS5weQ==) | `67.46% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `36.76% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_mmbt.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tbWJ0LnB5) | `17.6% <0%> (+17.6%)` | :arrow_up: |
| [src/transformers/tokenization\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fY2FtZW1iZXJ0LnB5) | `35.71% <0%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbmNvZGVyX2RlY29kZXIucHk=) | `25.3% <0%> (+25.3%)` | :arrow_up: |
| [src/transformers/data/metrics/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL21ldHJpY3MvX19pbml0X18ucHk=) | `26.66% <0%> (-1.25%)` | :arrow_down: |
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `93.93% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.72% <100%> (+87.72%)` | :arrow_up: |
| [src/transformers/tokenization\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fY3RybC5weQ==) | `96.11% <100%> (ø)` | :arrow_up: |
| [src/transformers/configuration\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2Rpc3RpbGJlcnQucHk=) | `100% <100%> (ø)` | :arrow_up: |
| ... and [75 more](https://codecov.io/gh/huggingface/transformers/pull/2545/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=footer). Last update [7833dfc...0e778f9](https://codecov.io/gh/huggingface/transformers/pull/2545?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).