Improve ListPtr::extract() (#21753)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21753
- it accidentally didn't move non-IValue-based lists before. This is fixed now.
- it only needs to recreate a T() for IValue-based lists
Reviewed By: resistor
Differential Revision: D15809220
fbshipit-source-id: 944badf1920ee05f0969fff0d03284a641dae4a9
