You can't clone a single file using git.
Git is a distributed version control system, the Idea behind its clone functionality is to have a complete copy of project and all versions of files related to that project.
Either download your file directly from here or clone the whole project using:
clone https://github.com/tensorflow/tensorflow.git
Also from here:
If there is web interface deployed (like gitweb, cgit, Gitorious, ginatra), you can use it to download single file ('raw' or 'plain' view).
If other side enabled it, you can use git archive's '--remote=' option (and possibly limit it to a directory given file resides in), for example:
$ git archive --remote=git@github.com:foo/bar.git --prefix=path/to/ HEAD:path/to/ | tar xvf -