Slight refactor of ETRecord Consumption (#1200)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1200

This diff makes 2 main changes:
- Updates `create_debug_handle_to_op_node_mapping` to construct the dictionary internally
- Moves ETRecord related logic in `Inspector's` constructor into a separate function

Reviewed By: Olivia-liu

Differential Revision: D51135613

fbshipit-source-id: 4f022b8d34d33eb1db5235f4611c5b1d26d1d38e