Add stitching utilities

[adamjstewart]

We should add a torchgeo.utils package that provides utilities for common operations, including stitching together patches to create a prediction on an entire tile. This is equivalent to torchvision.utils. See https://arxiv.org/pdf/1805.12219.pdf for a survey of common stitching techniques that we should support. This includes clipping (best default) and averaging. May also want to add weighted averaging. Let's see what libraries exist for this. If rasterio can do this for us for free, that would be awesome.

[Z-Zheng]

See here https://github.com/BloodAxe/pytorch-toolbelt