fix order of input/target of cross_entropy
https://pytorch.org/docs/stable/nn.functional.html#cross-entropy
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6642?src=pr&el=h1) Report
> Merging [#6642](https://codecov.io/gh/huggingface/transformers/pull/6642?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d0e42a7bed3de9271ae39c575d7eeb54cf985921?el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6642/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6642?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #6642   +/-   ##
=======================================
  Coverage   79.14%   79.14%           
=======================================
  Files         156      156           
  Lines       28248    28248           
=======================================
  Hits        22358    22358           
  Misses       5890     5890           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6642?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `82.45% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (+0.25%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6642?src=pr&el=footer). Last update [d0e42a7...502f692](https://codecov.io/gh/huggingface/transformers/pull/6642?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.
<|||||>As far as I can see it, this snippet in the Readme is not working correctly.