Detectron doesn't include torch as a formal dependency

[mdvthu]

Instructions To Reproduce the 🐛 Bug:

1. Full runnable code or full changes you made:

   cd new-python-project-requiring-detectron2
   echo 'detectron2 @ git+https://github.com/facebookresearch/detectron2.git#egg=detectron2@v0.6' >> requirements.txt
   python3.10 -m venv venv
   . venv/bin/activate
   python -m pip install -r requirements.txt

The above will fail as torch is not already available and not installed as a dependency

Desired behavior:

If detectron2 included torch as a stated dependency, it would make fresh install more straightforward. It currently crashes without a useful error message due to the import torch statement in setup.py

This limits the usefulness of using clean virtual environments (with repeatable requirements.txt) as a manual install of pytorch is required first

[Programmer-RD-AI]

An PR #5296 has been setup :)