host/load pretrained weights for 3D resnet

Is your feature request related to a problem? Please describe.
PR https://github.com/Project-MONAI/MONAI/pull/2253 implements a generic version of resnet for spatial 1/2/3D inputs. It'd be very useful for MONAI to provide a further pretrained=True option for the model initialisations. However there is currently some practical issue https://github.com/Project-MONAI/MONAI/pull/2253#issuecomment-851649305:

I was also wondering if I could address some issues I am currently facing with pretraining the network. Personally I would like to implement loading the state dict from the paper "Med3D: Transfer Learning for 3D Medical Image Analysis" (https://github.com/Tencent/MedicalNet), I think this closer resamples the issues monai tries to solve, and I receive better performances for my datasets with this pretrained network vs others (i.e. https://github.com/kenshohara/3D-ResNets-PyTorch).
Problem however is that medicalnet doesn't provide a url download for the state dict only, it only provides a downloadable, 2.7 Gb, zip file which include all pretrained networks and testing data. As I don't want to download the whole zip if someone wants to use a pretrained model, does monai have a zoo storage somewhere for pretrained weights for me to store individual state dicts? Or would you prefer if I would implement the pretrained weights on non medical images? Or no pretraining option at all?

cc @Douwe-Spaanderman

Start by reviewing PR #2253 and its linked discussion about pretrained 3D ResNet weights. Compare the proposed MedicalNet state-dict approach with the alternatives mentioned in the issue, then seek a maintainer decision on storage and scope. Done means an agreed implementation path for hosting and loading the selected pretrained weights.