make bundled program as an integrate class (#1616)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1616

Currently BundledProgram does not have its own ctor. To create a `BundledProgram`, we need to leverage `create_bundled_program` api.
To make the logic more concise, and integrate all bundled program AOT apis as an individual class (which for further api update), this diff removes `create_bundled_program`, and creates a ctor.

Reviewed By: Jack-Khuu

Differential Revision: D52832295

fbshipit-source-id: 66a7c3e78d976a242215d330a5d8c323bb737b63