Modification to PerceptualLoss to clarify when you can use 3D network medicalnet

[virginiafdez]

When you select medicalnet (either 10 or 50Resnet), and you use is_fake_3D = True, there is an LPIPS error coming out:

AttributeError: 'LPIPS' object has no attribute 'chns

There should be an error because is_fake_3D should be False if a 3D net is used, but the error doesn't show where the problem is.
I suggest we change the lines 52/53 in init:

if spatial_dims == 2 and "medicalnet_" in network_type: raise ValueError("MedicalNet networks are only compatible with ``spatial_dims=3``.")

to:

if (spatial_dims == 2 or is_fake_3D) and "medicalnet_" in network_type: raise ValueError("MedicalNet networks are only compatible with ``spatial_dims=3``. Flag is_fake_3D must be False.")