Make sure that MedicalNet can be only used in 3D cases inside the Perceptual Loss

[Warvito]

Currently, if we chose to use one of the medicalnet pretrained networks and set spatial_dims as 2D, it do not raise any error during the instantiation of the loss object. We should add this to avoid any problems.

Besides that, we could change from "medicalnet_..." to "mednet3d" (as suggested in #153 (comment)) to clarity that it is a 3D network.

[Warvito]

Since changing medicalnet to med3d will ned to change the hub repo, I will keep as medicalnet for now. The error raised in case the user select spatialdims==2 should be enough to avoid any problems.