sample weights

[samins]

This PR adds the option to add sample weights to model instances, by specifying weight_col to the dataset config. This creates a sample_weight: 1 in the feature map json. When this is absent, from now on all instances receive a weight 1 by default, since pytorch enforces DataLoader to output tensor so we cannot set the weights to None. We also set the package dependency to torch=1.7 the last version that supports fft as used now by the HolographicInteractionLayer.