__version__ assignment breaks local build.

[JackTemaki]

As the environment I am using has to be created at a time where a GPU is not available, and warp-rnnt does not allow installation without a GPU, I decided to compile it locally at runtime.

The problem is that if I build it locally with python setup.py build_ext --inplace, there is no distribution information, so it crashes at https://github.com/1ytic/warp-rnnt/blob/master/pytorch_binding/warp_rnnt/init.py#L6 with: DistributionNotFound: The 'warp_rnnt' distribution was not found and is required by the application

Of course I can create a local patchfile to remove that line manually to make it work, but maybe there is a more clean way to fix it here in the repo, e.g. by setting a __version__ = "unknown" or something when an exception is thrown.