Shuffle train subset for summarization example
Fix #3892 
@sshleifer 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=h1) Report
> Merging [#3909](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cb3c2212c79d7ff0a4a4e84c3db48371ecc1c15d&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3909/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3909      +/-   ##
==========================================
+ Coverage   78.45%   78.47%   +0.01%     
==========================================
  Files         111      111              
  Lines       18521    18521              
==========================================
+ Hits        14531    14534       +3     
+ Misses       3990     3987       -3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3909/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3909/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.92% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3909/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `68.86% <0.00%> (+0.36%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=footer). Last update [cb3c221...3361612](https://codecov.io/gh/huggingface/transformers/pull/3909?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
