GitLab

Summary:
We now require CUB for building, here we make the tutorials include it.

Also make the installation cell do nothing if it has already succeeded.

I use curl not wget, and `os.environ` to set the variables not shell methods, because they are more likely to work on Windows.

Reviewed By: nikhilaravi

Differential Revision: D24860574

fbshipit-source-id: 5be86af15e53f8db016ee0e96fb43153bd69adbc