[Instance segmentation model] Please implement Mask R-CNN model family

[cregouby]

Mask R-CNN is a instance segmentation model is based on the Mask R-CNN paper

Please implement

• maskrcnn_resnet50_fpn
• maskrcnn_resnet50_fpn_v2

ℹ️ The pretrained model weights have been uploaded to https://torch-cdn.mlverse.org/models/vision/v2/models/maskrcnn_resnet50.pth and https://torch-cdn.mlverse.org/models/vision/v2/models/maskrcnn_resnet50_v2.pth