Filter out a UserWarning in torch-1.9
Filter out a UserWarning in torch-1.9

Summary:
Filter out the following `UserWarning` triggered by the floor division in the original code when using pytorch-1.9.0. Replace with the [torch.div()](https://pytorch.org/docs/stable/generated/torch.div.html#torch.div) API as suggested.

```
UserWarning: floor_divide is deprecated, and will be removed in a future version of pytorch. It currently rounds toward 0 (like the 'trunc' function NOT 'floor'). This results in incorrect rounding for negative values.
To keep the current behavior, use torch.div(a, b, rounding_mode='trunc'), or for actual floor division, use torch.div(a, b, rounding_mode='floor'). (Triggered internally at  /opt/conda/conda-bld/pytorch_1623448224956/work/aten/src/ATen/na
tive/BinaryOps.cpp:467.)
```

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Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/3469

Reviewed By: wat3rBro

Differential Revision: D31953626

Pulled By: ppwwyyxx

fbshipit-source-id: 2b91b396dc62612dfb7aa9e55c41f32b83233756

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2