Summary:
there are several imports within the `sam3.model` package that reference other packages within `sam3` other than `sam3` and `sam3.model` (for example [here](https://github.com/facebookresearch/sam3/blob/main/sam3/model/sam3_tracker_base.py#L15)). This fixes the package structure so that you can `pip install` the package and `import sam3`
Pull Request resolved: https://github.com/facebookresearch/sam3/pull/327
Reviewed By: haithamkhedr
Differential Revision: D88950127
Pulled By: lematt1991
fbshipit-source-id: 3554512d304ccdf679a9af8606bbfe1f7f2a1cfb
