Fix validation of dynamic axes names (#23974)
Summary:
Existing code adds two enumerators to the set instead of forming their union.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23974
Differential Revision: D16732762
Pulled By: ezyang
fbshipit-source-id: 787737b7cf4b97ca4e2597e2da4a6ade863ce85c
