Adding support for a scale factor when parsing ETDump results (#534)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/534

By default, event profiling time is treated as ms (1/1000), but users should have the ability to define the scale

Note that the default is set to 1000 only in the constructor. Private functions default to no-op (scale = 1)

Reviewed By: tarun292

Differential Revision: D49762707

fbshipit-source-id: 7f1191d163a2e0aff1005daf74dfda3a3d45eaea