Detectron2 + PyTorch CPU : Fix Install issue

Detectron2 + PyTorch CPU

❌ The error

$ pip install -q --extra-index-url https://download.pytorch.org/whl/cpu torch torchvision

$ pip install --extra-index-url https://download.pytorch.org/whl/cpu \
             "git+https://github.com/facebookresearch/detectron2.git"

Collecting git+https://github.com/facebookresearch/detectron2.git
  Cloning https://github.com/facebookresearch/detectron2.git ...
  Running command git clone --filter=blob:none ...
  Resolved https://github.com/facebookresearch/detectron2.git to commit d38d716...
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error

  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [20 lines of output]
      ...
      ModuleNotFoundError: No module named 'torch'
      [end of output]

✅ The solution

pip install --no-build-isolation \
            --extra-index-url https://download.pytorch.org/whl/cpu \
            "git+https://github.com/facebookresearch/detectron2.git"

Why it works
--no-build-isolation tells pip to reuse the packages already in your virtual-env instead of creating a fresh, empty build sandbox.