improve error message on recursive class defs (#21842)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21842
ghimport-source-id: 33569714b18fc476c4e6b3bc976b53b1f107273d
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D15857568
Pulled By: suo
fbshipit-source-id: 6307597b9741cfdccd5c55216ebdc7c4391a5e23
