The weights are not for the entire MoCo model (i.e. with encoder, momentum encoder, and queue) but just for ResNet50. Have a look at how they load the weights for ImageNet classification: moco/main_lincls.py at 78b69cafae80bc74cd1a89ac3fb365dc20d157d3 · facebookresearch/moco · GitHub 31
In this case model = torchvision.models.resnet50()