In the function visualize_and_dump in p3i\utils\vis.py instead of generating the output folder with:
os.system(f'mkdir -p {args.output_dir}')

try to use the available dedicated function os.mkdirs:
os.makedirs(args.output_dir, exist_ok=True)

On my Windows 10 machine the former command had the side effect of generating a folder named '-p', as I think on Windows the command mkdir does not accept any parameter (https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/mkdir).

In the class P3IOptions, the default value for the argument output_dir is currently:
./results
but should be:
results

With the former, the commands os.path.join(args.output_dir, ...) in the function visualize_and_dump (in p3i\utils\vis.py) break on Windows 10, as ./results is a Unix path not valid on Windows.

I have went through all the code and still not found some code like loss.backward(). So how does p3i do the gradient descend?

In the installation step, the package 'tqdm' is missing in the last step.
Instead of:
conda install -y pillow opencv
it should be:
conda install -y pillow opencv tqdm

This package is used in the file program/p3i.py.