WIP: Use appropriate escaping for the torch install script

I think there is a problem with the escaping in the PyTorch install script.

The use of single quotes when generating the file to set the environmental variables means that the $(pwd) commands do not get executed and get written literally in the file. As a result, the source_to_run_pytorch file only works properly if you source it while in the same directory as the source file. I am not sure whether sourcing from the same directory is the desired behavior, but this change removes that constraint.

I have only done one of the different installation routes, but can do the others if you think this change is appropriate.