Dict mutability (#16884)
Summary:
Adds `aten::_set_item` for `dict[key]` calls
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16884
Differential Revision: D14000488
Pulled By: driazati
fbshipit-source-id: ea1b46e0a736d095053effb4bc52753f696617b2
