Create parser.cpp (#15238)
Summary:
Moves implementation into .cpp file. Parser was getting included in several compilation units.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15238
Differential Revision: D13474635
Pulled By: zdevito
fbshipit-source-id: 7dc824eea8f506d6c8ae1aa67aeec0c34d5285fc
