note on updating existing source (#18409)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/18388
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18409
Differential Revision: D14597666
Pulled By: soumith
fbshipit-source-id: 156104c0cd19da06f6f96a225228d1e8cf831af1
