Add etdump_gen to generate flatbuffer containing profiling/debug events (#338)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/338

1. generate etdump
2. deserialize & verify the values
3. check for edge cases

There is a TODO in here once the `EventTracerEntry` struct has been modified to include the updates to the etdump flatbuffer schema or devise a way to evaluate the necessary information from the existing struct.

Reviewed By: tarun292

Differential Revision: D48743211

fbshipit-source-id: 35327250efe9150d0ddcebd56b6b5b5fd3d53e8d