How to install a virtual environment of Python in macOS
I would install another python as this will allow you to put new packages in  consistent places. Apple's python as you note is missing pip. (and does not put packages where your linked page says) (Although the web site you show does have a page telling you how to install pip)

I would also use python3 unless you have a good reason not to. Which again means install another python. 

You can either install plain python from https://www.python.org/ This python includes pip but that needs to be upgraded as per pip's website This installation will have to be done under sudo.

Otherwise you can use a packaged solution e.g. Macports or Homebrew - which I would use if you want a lot of C libraries or like Anaconda which is just a python packager.

(I  would normally say that Anaconda is the best for new users as it includes more ) However CUDA is low level code and the USEAGE for the FastPhotoStyle does say that it has only been tested under OS: Ubuntu 16.04 So I would not expect it to run under MacOS.