How can SAM2 be used on Macs with Apple Silicon?

I downloaded the repo and ran `SAM2_BUILD_CUDA=0 pip install -e .` but when I try using the following (super) minimal test script, it fails with `RuntimeError: Placeholder storage has not been allocated on MPS device!`. This is being caused by the last line of the example.

```
import torch
from sam2.build_sam import build_sam2
from sam2.sam2_image_predictor import SAM2ImagePredictor
```

https://github.com/facebookresearch/sam2/issues/122 says it supports MPS, but it doesn't work for me. What are the steps to get a minimal example of SAM2 running on a Mac? My Mac is an M4 if that matters.