[pruning] Speedup activation reconstruction (#60683)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60683
Vectorized reconstruction without for loops
Test Plan:
`buck test mode/dev-nosan //caffe2/test:ao -- TestBasePruner`
https://pxl.cl/1KSJQ
Reviewed By: z-a-f
Differential Revision: D29370805
fbshipit-source-id: 75402437654a0b6f6391c8590bbe3f6fe3f43d8f
