Add polygon dataset class

[Om-Doiphode]

Description

This PR adds a polygon dataset that can be used for training segmentation models like Mask RCNN.

1. Create a polygon dataset class.
2. Add MaskRCNN to deepforest/models/ directory.
3. Confirm that preprocess.split_raster and other utilities/visualization functions can take in polygon predictions.
4. Create an example IPython notebook showing how to train a polygon model, train an initial model using open-source polygon data

Related Issue(s)

Fixes issue #758

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[jveitchmichaelis]

Could you scope this pr to only the dataset/loader please? You could have a look at the keypoint commit (recently) for reference.

We can test that we can evaluate predictions (would need to add some to the repo) without integrating the training and inference loop.

We should also think about:

• What format labels do we support loading? I think we probably should support MS-COCO at this point, or at least add a conversion script to generate a suitable CSV with the correct geometry.
• Output format (get_item)? Typically this is boolean mask-per-object or it may need to be type long for torchvision.
• Similarly, format convention for models in HuggingFace? See MaskFormer, etc.