FCCD Models refactor

[isaaccorley]

This PR;

• removes FCEF model since this is basically a U-Net with the 2 change images stacked together so the input channels are 2*num_channels.
• Refactors FCSiamConc and FCSiamDiff to inherit from segmentation_models_pytorch.Unet. These models are basically U-Nets but feed both images through the U-Net encoder and then either take the difference (FCSiamDiff) or concatenate (FCSiamConc).

Notes:

• By inheriting from smp this allows us to easily load different encoder backbone weights (I'll add a follow up PR to add this later). Previously the backbone was fixed.
• For the models I'm reusing the smp.Unet constructor and it's args. For FCSiamDiff I made a dummy constructor so that I had a place to put the docstring. Not sure if this is the best way.

[isaaccorley]

@adamjstewart I'm getting the below errors in the docs. Do you know if there's a quick fix for these? I'm basically inheriting from some segmentation_models_pytorch classes and I think it's unable to find information from the parent classes.

generating indices... /home/docs/checkouts/readthedocs.org/user_builds/torchgeo/checkouts/345/torchgeo/models/__init__.py:docstring of torchgeo.models.FCSiamConc:1: WARNING: py:class reference target not found: segmentation_models_pytorch.base.model.SegmentationModel
/home/docs/checkouts/readthedocs.org/user_builds/torchgeo/checkouts/345/torchgeo/models/__init__.py:docstring of torchgeo.models.FCSiamDiff:1: WARNING: py:class reference target not found: segmentation_models_pytorch.unet.model.Unet

[adamjstewart]

@isaaccorley you need to add smp to the intersphinx_mapping in docs/conf.py.

[isaaccorley]

Still can't seem to get this one to work. Wonder if it's similar to your issue with the PyTorch modules.

[calebrob6]

Cool should be good now

[adamjstewart]

When I make the 0.3.0 release notes, remind me to add this FCEF removal as a backward incompatible change in case anyone was relying on it.

Also, who was it who added these models? Should we ping them to review? (guess we should've done that before merging but it's not too late to undo anything if it's problematic)