Make package available globally when installed

[tnwei]

Hi! Made a little edit in setup.py such that the package is available globally when installed.

Instead of doing this in the diffusion notebooks:

!git clone https://github.com/crowsonkb/guided-diffusion
!pip install -e ./guided-diffusion
sys.path.append('./guided-diffusion')

You can do without manually adding the folder to path, simplifying to just this:

!pip install git+https://github.com/crowsonkb/guided-diffusion