DataModule: common base class to reduce code duplication

[adamjstewart]

A lot of the code in our base classes was shared, so I aggregated it.

Originally I was just planning on adding better error messages, but figured I would do this first.

[calebrob6]

This feels kind of like premature optimization (is premature abstracting a thing?) as it only saves a net few lines (just repeated twice), and "DataModule" doesn't seem like the correct name. It is good because I can't think of a case why we would want to go back and make different plot, on_after_batch_transfer or prepare_data methods for the Geo and NonGeo data modules.

[calebrob6]

Should we call this a TorchgeoDataModule or something?

[adamjstewart]

Could, I don't think it matters too much. The user should never use this class.

[adamjstewart]

For context, the reason I made this change was because I wanted to improve the datamodule error messages. The current error message just says:

Foo.setup does not define a 'foo_dataset'

but the actual error could be any of:

• Foo.setup does not define a foo_dataset or dataset
• Foo.setup does not define a foo_sampler or sampler or foo_batch_sampler or batch_sampler
• The dataset length is 0
• The sampler length is 0

Since this is a bit of code and requires access to self, I figured it would make sense to add this check to a base class. When I started working on a base class, that's when I realized how much duplicated code there was. So I figured it would be easiest to add the base class in one PR and improve error messages in a second PR.

[calebrob6]

Totally fine! I just think the name "DataModule" might be confusing, maybe "BaseDataModule"? Else, I'm cool with it.