Models are missing `model_transforms` property

As of now, models are missing a model_transforms property, one of them being the resizing to the model input size.

I'm not sure how the examples with Pasa, Densenet and Alexnet are working right now, but this is definitely something we want to have around.

The model_transforms provided by the models should be set on our own CachingDataModule.

Done in 16087330

model_transforms are defined in each model's __init__(). When configuring a datamodule, for example in train.py, we access the transforms from the model and add them to the datamodule.

This has to be done before calling datamodule.setup()

This change allows us to get rid of "rgb" dataset config files, since the conversion is done automatically depending on the model.

Note that depending of the dataset and model combination, some images could be loaded as RGB, then "converted" to RGB which is redundant. Same thing if the loaded images are already of the correct size for the model input, there will be a redundant resize transform applied.

The user should always carefully read each dataset and model description and make sure to understand what the transform pipeline does. A greyscale dataset used with an RGB model is possible, but the color channels will simply be the greyscale data replicated three times. This works without raising any error or warning.

@dcarron: we introduced some more changes that affect this. The Pasa model was ported, but we still miss an equivalent action on Densenet and Alexnet models. The current implementation of the transforms on these models is robust, but requires 2 memory copies we could probably reduce to 1, if we follow the same principle of the Pasa model transforms.

@biosignal: Whomever wants to work on this, please drop a line here, or at Mattermost.

I'm doing this now.

mentioned in commit 6c981135

This is done with the commit above. This issue will be closed when the main merge request gets accepted.

mentioned in commit 60e74703

closed with merge request !6 (merged)

mentioned in commit adf29dd1