INSTALL.md is not up-to-date. Here is an updated version in my SlowFast fork

Hello,
INSTALL.md file is not up-to-date and has below issues:

- torchvision with ffmpeg support (video_probe_from_memory errors),
- Pytorchvideo: pip install lacks some necessary functions,
- sklearn not supported anymore, scikit-learn can be explicitly installed,
- python version: python 3.9 not supported for building torchvision with ffmpeg,
- order of installation steps is confusing, etc.

I included more details about the above problems and their solutions in my own SlowFast fork in the [INSTALL.md file here](https://github.com/alpargun/SlowFast/blob/main/INSTALL.md).
I hope these changes in the installation steps help you build and work with SlowFast easier.