Make nn.Sequential iterable (#28987)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28987
We have `__iter__` defined on nn.ModuleList. Chainer's `Sequential` defines `__iter__`. This will also be helpful in modules which extend `nn.Sequential` and define a custom forward, because they can use the `for x in self` syntax that is supported in both python & TorchScript.
Test Plan: Imported from OSS
Differential Revision: D18402822
Pulled By: eellison
fbshipit-source-id: 1ece0f891a9d37f401e232320f58b056d5481856
