Introduce GraphBuilder abstraction (#2061)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2061

## Context

This changeset introduces `VkGraphBuilder` to handle parsing an `ExportedProgram` and parsing it to construct a `VkGraph` for serialization. Most of the graph parsing functionality previously implemented in `vulkan_preprocess` is now in `VkGraphBuilder`.

The main motivation of this refactor is to simplify `vulkan_preprocess`.
ghstack-source-id: 216267433
exported-using-ghexport

Reviewed By: manuelcandales

Differential Revision: D54128091

fbshipit-source-id: f0670e76719f9bff1212c1286bd127ef4e37b94d