Fix time scale conversion in inspector (#860)

Summary:
Having a source and target time scale is better so that we don't bake in too many assumptions into the API and provides more flexibility to the users.

As we have no external users yet using this API it's a low risk change.

Pull Request resolved: https://github.com/pytorch/executorch/pull/860

Test Plan: CI

Reviewed By: Jack-Khuu

Differential Revision: D50208476

Pulled By: tarun292

fbshipit-source-id: 2aa505211ec723ba49a202d2229b4e188a4bbe85