Update to PyTorch and PyG LTS releases

• Update PyTorch to 1.8.2 LTS
• Update pytorch-geometric to 1.7.2
• Rewrite message passing to use aggregate(...) instead of overwriting
  propagate
• Remove PCA dimensionality reduction and use learnable MLP to reduce
  node feature dimensionalityd

Pre-trained models

Add pre-trained models

• pca.sklearn is used for dimensionality reduction of visual features in pre-processing script
• faster_rcnn_fpn.model is the object detection model
• graph_nn_bce_30.pth is the trained GNN model for tracking