Make edge dialect program a first class citizen (#508)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/508

As building out the Inspector APIs, we realized that we want to make edge dialect program a first class citizen of ETRecord, and also require executorch program when constructing an ETRecord (used to be optional).

Reviewed By: tarun292

Differential Revision: D49436040

fbshipit-source-id: 59986e75d3da141502066d152925119998426d4e