Use `math.pi` instead of `torch.pi` in `MaskFormer`
# What does this PR do?

`MaskFormer` uses `torch.pi` which is only available in `torch >= 1.10`. The Past CI with PyTorch 1.9 gives
```bash
AttributeError: module 'torch' has no attribute 'pi'
``` 
This PR adds a warning if PT <= 1.9 for this model, similar in `ViltModel` https://github.com/huggingface/transformers/blob/71fc33174664738d8c8d93025ebc810180e69c20/src/transformers/models/vilt/modeling_vilt.py#L44


**Question: Should we also skip the whole test suite for  `MaskFormer` depending on the torch version? The objective is to make Past CI cleaner if we ever run it again with previous torch version.**
_The documentation is not available anymore as the PR was closed or merged._<|||||>> If it's just for `torch.pi` can we change this to `math.pi` and have the model work with PyTorch 1.9?

I can change it to `math.pi`.