Allow `neg_ratio` to be None in `ObjectDetectionGeoDataWindowConfig`

[AdeelH]

🚀 Feature

See the following discussion on Gitter: https://gitter.im/azavea/raster-vision?at=6298bff6ef00bd1dc60fc7ee

ObjectDetectionGeoDataWindowConfig should allow neg_ratio to be None since ObjectDetectionRandomWindowGeoDataset allows it.

Motivation

It's a feature that already exists. This will allow users to actually make use of it.

Pitch

ObjectDetectionGeoDataWindowConfig.neg_ratio's type hint will need to be changed to Optional[float].

Alternatives

N/A

Additional context

N/A