There is not a way to use torch.no_grad and find_unused_parameters=False since these operations are still included in the autograd graph, but only require_grad is set to False. See the find_used_parameters argument for DDP DistributedDataParallel — PyTorch master documentation 6
When you are detaching anchor_supports and target_supports then the autograd graph will not be tracking those operations, which is why the example you gave is working with find_unused_parameters=False.