Fix RuntimeError on generation_utils.py
This PR fixes a below runtime error on generation_utils.py

```
RuntimeError: Integer division of tensors using div or / is no longer supported, and in a future release div will perform true division as in Python 3. Use true_divide or floor_divide (// in Python) instead.
```
Think https://github.com/huggingface/transformers/pull/15498 superseeds this PR a bit as it corrects all occurances of PyTorch's integer divison. Hope it's ok if we go for https://github.com/huggingface/transformers/pull/15498 instead :sweat_smile: 