Enable Torchvision resnet50 on PerceptualLoss

[yiheng-wang-nv]

So far, PerceptualLoss does not support standard (torchvision based, https://pytorch.org/vision/main/models/generated/torchvision.models.resnet50.html) resnet50.
I will try to enable it. It would be helpful to use it as well as customized pretrained weights input.