Handle None indexing in TorchScript (#18615)
Summary:
t[None], t[None, 1:, None] and friends for unsqueezing
Fixes: #12810
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18615
Differential Revision: D14837039
Pulled By: wanchaol
fbshipit-source-id: ab3862c41629f087b0a46b7c59c93dac4018e6fe
