Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21177

- Integrate c10::ListPtr into IValue and the c10 dispatcher.
- Streamline conversion to/from IValue. Before, we had IValue::to<> and kernel_functor.h had its own ivalue_to_arg_type and return_type_to_ivalue. They are now unified. Also, this means that nested types like Dicts of Lists of Optional of Dict of ... do work as expected now

Differential Revision: D15476433

fbshipit-source-id: bde9df80df20091aa8e6ae17ba7e90abd149b954