Add get_inputs to method.cpp (#3775)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3775

There are use cases where users would like to access the input EValues after they've been memory planned and allocated buffers so that they can directly copy their inputs to these buffers. Adding a method to support that.

Reviewed By: JacobSzwejbka

Differential Revision: D57970255

fbshipit-source-id: 5fbcccdeda197d07b59cfdd71a6e35ac410b3b0e