Allow default construction of Dict/List (#22084)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22084
For DictPtr/ListPtr, default construction was disallowed because it was ambigious if it's supposed to create an empty list or a nullptr.
But since we renamed them to Dict/List, we can now allow default construction without ambiguity.
Differential Revision: D15948098
fbshipit-source-id: 942a9235b51608d1870ee4a2f2f0a5d0d45ec6e6
