Summary:
Running code that defines classes using `torch.class` in an interactive environment leads to the well-intentioned but undesirable `parent class/factor is already defined` error.

This PR introduces a hacky workaround that silently unsets the existing class if it already exists.
Closes #136

Reviewed By: soumith

Differential Revision: D3982016

Pulled By: ebetica

fbshipit-source-id: 4fa402ee9254e9127025910ea0ef9a4ea6535a55