New util function for getting delegation summary (#2611)

Summary:
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Pull Request resolved: https://github.com/pytorch/executorch/pull/2611

Because what happened to the graph in to_backend() is pretty opaque to the users, the existing [print_delegated_graph()](https://fburl.com/code/4xa5oewv) is good but might be too much for new users, so implemented this new util function to give a more lightweight summary for the delegated graph.
Original design doc:
https://docs.google.com/document/d/19ZSDddm23MnGvFUrkV9clwHwLvXzAo2IZtbkr9VChjE/edit?usp=sharing

**Note:** The added `import pandas` shouldn't be an issue because we already have pandas in the install requirement is executorch oss: https://fburl.com/code/p55654r2

Reviewed By: dbort

Differential Revision: D55239751

fbshipit-source-id: 81000953b51d87f6d509a6589685fe677a8eec5b