add support for ModuleDict (#25715)
Summary:
Add support for nn.ModuleDict in script. This is needed to support torchvision.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25715
Differential Revision: D17301826
Pulled By: eellison
fbshipit-source-id: 541b5477e980f519a8c3bbb1be91dac227f6d00f
