Move AutoGraph to core. This CL moves the entirety of the code base, keeping...

Move AutoGraph to core. This CL moves the entirety of the code base, keeping the frontend autograph module in contrib for backward compatibility. Certain files, like notebooks and the readme file may be referenced from the outside, so a copy of those is kept as well. In addition, the notebooks subdirectory of examples is also kept in contrib because the extension the build file relies on is not available in the PIP package.

PiperOrigin-RevId: 212543067