How to build open3d.ml in Mac OS?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Steps to reproduce the issue

problems

### Error message

```shell
Pytorch 1.8 is not available in Mac os
```

### Open3D, Python and System information

```markdown
0.15.1
```

### Additional information

_No response_