Drop `base_folder` from datasets about torchgeo

[adamjstewart]

We could also do this only for GeoDatasets which are more likely to already be present on a system like Azure/AWS/GCP.

[adamjstewart]

A more concrete example of why this is useful:

The planetary computer comes with NAIP data in the following directory structure: https://planetarycomputer.microsoft.com/dataset/naip#Storage-Documentation

Because that directory structure doesn't contain a naip directory, it wouldn't be possible to use it with TorchGeo if we used base_folder = "naip".

[adamjstewart]

Another reason for this change:

It should be easy to use TorchGeo without reading the source code. If I want to download a dataset myself, I currently can't use it without reading the source code and figuring out which subdirectory of root the data is expected to be found in. I'm required to use this exact name, and to pass the parent directory of this subdirectory as root. It's much more intuitive to me for root to be the directory where the data is found, not the parent directory.