Improve data module error messages

[adamjstewart]

Summary

We should improve our data module error messages by being more specific about what issues a data module may have.

Rationale

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

We should explain which of the above is actually happening.

Implementation

We added a new BaseDataModule class. We can add a method to that which will be inherited by subclasses. This method will be given a list of either datasets or sampler, then check to see if at least one of them is defined and has length > 0, and prints the appropriate error message if that is not the case.

Alternatives

No response

Additional information

No response