[FIX] fix repetition penalty in `examples/run_generation.py`
The repetition penalty in `examples/run_generation.py` is incorrectly implemented due to the following snippet.

```python
            for _ in set(generated):
                next_token_logits[_] /= repetition_penalty
```

`generated` is a tensor, and python built-in `set` does not compare tensors correctly, e.g.:

```python
>>> import torch
>>> set(torch.cat([torch.arange(2),torch.arange(3)]))
{tensor(0), tensor(1), tensor(1), tensor(0), tensor(2)}
```

This PR fixes this subtle error.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1551?src=pr&el=h1) Report
> Merging [#1551](https://codecov.io/gh/huggingface/transformers/pull/1551?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c5441946112e68441b46866d114bf8d3c29b0c1d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1551/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1551?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1551   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files          91       91           
  Lines       13593    13593           
=======================================
  Hits        11713    11713           
  Misses       1880     1880
```



------

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<|||||>Hello! That's great, thank you!