Fix Swin ONNX export warnings
# What does this PR do?

This PR fixes uninformative warnings in the ONNX export for Swin as reported in https://github.com/huggingface/transformers/issues/19780 , namely

```
UserWarning: __floordiv__ is deprecated, and its behavior will change 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').
```

Since  `torch.jit.trace` assumes expressions like `tensor.size(0)`, `tensor.size()[1]`, `tensor.shape[2]` are tensors in tracing mode ([reference](https://ppwwyyxx.com/blog/2022/TorchScript-Tracing-vs-Scripting/)), the warning is raised.

## Before submitting
- [x] This PR fixes a typo

## Who can review?

@lewtun 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Note that this will only work in recent versions of PyTorch. As was done for the [int div](https://github.com/huggingface/transformers/blob/321ef388fe041f630e65abc26a3be8580d7e858b/src/transformers/pytorch_utils.py#LL35C3-L35C3), you should create a util function in the PyTorch utils that works across PyTorch versions<|||||>Good to know, thanks a lot! I think there are other models having the issue of using `torch.div` then, I'll fix as well. I'm a bit skeptic about readability (including this PR), trying to support tracing overall makes the code less readable I feel.<|||||>Just realized that the warning above is not shown anymore in PyTorch 1.13. Seem to be https://github.com/pytorch/pytorch/pull/78411

Refer to the release notes: `Updated torch.floor_divide to perform floor division` https://github.com/pytorch/pytorch/releases

This is fine since it influences only negative numbers. I'll close this PR.