DataModules: rename split parameters

[adamjstewart]

Summary

We should rename the split parameters we use in data modules.

Rationale

Currently, our modules use val_split_pct and test_split_pct. However, our splitting functions only accept integer lengths or ratios, none of them actually accept percentages.

Implementation

We should use "length" to be consistent with torch.utils.data.random_split. We should either use lengths for methods that accept a variable number of splits or val_length/test_length for methods that differentiate between the two.

Alternatives

Open to suggestions for other names, but want to follow existing standards when possible.

Additional information

No response

[isaaccorley]

torchgeo.datamodules.utils.dataset_split accepts percentages. Also GeoDataModule already has a length argument.

[adamjstewart]

torchgeo.datamodules.utils.dataset_split was removed in #2005.

My suggestion is to replace all *_split_pct parameters with a single split_lengths parameter that gets passed to torch.utils.data.random_split. We can also rename length to sampler_length for clarity if we want to. This would be a backwards incompatible change though.