Install ninja-build with apt-get

[goldsborough]

@ezyang @zdevito

A braindump of how I got here:

Why do we need ninja

First, let me say that there are two ninja things:

1. ninja, the command line tool and build system,
2. ninja, the Python module that exposes a tiny module to emit ninja build files.

The PyTorch CI currently requires neither, however the new C++ extensions do require the command line tool (but not the Python module).

Why can't we pip install ninja?

It is currently very hard to install ninja with pip, as there is a bug in setuptools, that manifests itself like this:

pip install ninja
Downloading/unpacking ninja
  Downloading ninja-1.7.2.post2.tar.gz (295kB): 295kB downloaded
  Running setup.py (path:/tmp/pip_build_root/ninja/setup.py) egg_info for package ninja
    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/tmp/pip_build_root/ninja/setup.py", line 7, in <module>
        from skbuild import setup
    ImportError: No module named skbuild
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 17, in <module>

  File "/tmp/pip_build_root/ninja/setup.py", line 7, in <module>

    from skbuild import setup

ImportError: No module named skbuild

Sometimes, this can be solved by upgrading pip (pip install --upgrade pip) -- but not always (e.g. does not fix the problem on nightly Python). I have spent many hours trying to work around this.

The solution

The best solution IMO is to simply install ninja with apt directly, then there need be no pip issues.

This will not interfere with the rest of the build, since the ninja Python module is not installed by apt, so Pytorch's setup.py will not pick it up and still build with cmake. However ninja will be available as a command-line tool, so that the C++ extensions can be built.