This pull request:

• Updates all DeepLabCut example Jupyter Notebooks for use with the PyTorch engine
• Updates all COLAB Notebooks for use with the PyTorch engine
  • Installation of DeepLabCut assumes the pytorch_dlc branch was merged into main or a 3.0 release of DeepLabCut was made to pypi (depending on the notebook). Until that's the case, !pip install "deeplabcut>=3" need to be replaced with !pip install "git+https://github.com/DeepLabCut/DeepLabCut.git@pytorch_dlc#egg=deeplabcut and !git checkout pytorch_dlc needs to be run in COLABs where the DeepLabCut repo is cloned.
  • The COLAB_3miceDemo.ipynb notebook is updated to point to other notebooks and keeps its TensorFlow engine
  • COLAB_maDLC_TrainNetwork_VideoAnalysis.ipynb is renamed to COLAB_YOURDATA_maDLC_TrainNetwork_VideoAnalysis.ipynb as it is intended to be used with users' data
• Updates the deeplabcut/create_project/demo_data.py script to set the Engine to PyTorch

Notebooks were tested locally, and COLABs were tested on Colab.