Use chain.from_iterable in optimizer.py (#40156)
Summary:
This is a faster and more idiomatic way of using `itertools.chain`. Instead of computing all the items in the iterable and storing them in memory, they are computed one-by-one and never stored as a huge list. This can save on both runtime and memory space.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40156
Reviewed By: ezyang
Differential Revision: D22189038
Pulled By: vincentqb
fbshipit-source-id: 160b2c27f442686821a6ea541e1f48f4a846c186
