PyTorch implementation of block non-local operation in paper "Non-Local ConvLSTM for Video Compression Artifact Reduction"

It just appears to me that the authors never upload the core code of the proposed block non-local operation and I happened to implement this part of code in PyTorch quite long time ago. I think it's time to share with others. I respect the authors that proposed such good idea so I'm gonna send the PR to the original repo. I guess this is the correct implementation.