Introduce DelegateMappingBuilder (#184)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/184

For creating the debug_handle_map generated in the design doc, this diff introduces a helper class for manage the state of the handle map

`get_delegate_mapping`: Is used to return the debug handle map out of preprocess
`upsert_delegate_mapping_entry`: Handles the creation and manipulation of debug handle entries
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Reviewed By: tarun292

Differential Revision: D48569900

fbshipit-source-id: 2a6e462d9306380465a46ae20b2aeb0cb1c4dc67