Summary: This seems to not happen reliably and causes rss to grow. The `__dealloc__` cannot fix it completely, although I verified it is called. So calling it explicitly seems to guarantee it to happen.

Reviewed By: yfeldblum

Differential Revision: D20206887

fbshipit-source-id: 6285a8e8943a53d20714e54efd3220a545010102