Fix Windows-incompatible mkdir and add COCO auto-download to tutorial notebook

[ashishsoni-ai]

What

Fixes tutorial_image_detection.ipynb so it runs end-to-end on Windows without manual intervention.

Why

• !mkdir -p fails on Windows (cmd.exe doesn't support the -p flag), throwing a SyntaxError
• The COCO dataset cell had placeholder paths (<PATH_TO_COCO_VAL2017_IMAGES>) with no download guidance, blocking new contributors from running the notebook at all

Changes

• Replaced the Unix-only !mkdir -p calls with cross-platform os.makedirs(..., exist_ok=True)
• Added a cell that automatically downloads and extracts COCO annotations, with an option to either:
  • use a small subset (default, ~20 images, fast) for quick testing, or
  • download the full val2017 dataset for complete evaluation
• Updated the dataset-loading cell to use the downloaded paths instead of placeholders

Testing

Ran the full notebook end-to-end on Windows 11 (Python 3.11, PyTorch 2.4.1 CPU) — all cells execute successfully, including model inference, evaluation metrics, and the precision-recall curve output.

Fixes #612

[dpascualhe]

Hi @ashishsoni-ai , nice contribution! The only thing that needs fixing before merging is removing duplicated imports and moving those that are actually new to the first cell for consistency

[ashishsoni-ai]

Thanks for the review @dpascualhe . Cleaned up the duplicate imports and moved the new ones into the first cell. Let me know if anything else needs adjusting — happy to help with more issues if there are any you'd point me toward.