Make torch_xla package PEP 561 compliant (#9515)
The repo seems to be using type hints very thoroghly, this PR adds
changes to make the package PEP 561 compliant:
https://peps.python.org/pep-0561/
This avoids errors like the following when using mypy:
```bash
infinity_emb/inference/loading_strategy.py:13: error: Cannot find implementation or library stub for module named "torch_xla" [import-not-found]
```
Torch package does the
[same](https://github.com/pytorch/pytorch/blob/8e07c9870d07c5a318ab21bb16b3fa27576851e6/setup.py#L1288)
