Reserve vectors that we know the size in advance for. (#16201)
Summary:
Save reallocation costs, by reserving vectors according to how many elements we expect to put in.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16201
Differential Revision: D13762594
Pulled By: ezyang
fbshipit-source-id: 7e3bfe421489dde48a2ddb0920dd155f69baecc0
