Clean up timing class.
Clean up timing class.

* Remove the 'core_hours_approx_v3' logging as the used heuristic conversion can be really off.
* Remove 'epoch'. It's not really usable.
* Add `self.warmup` for starting the timing.
* Set `self.prev_step = first_step`. [We used to set it to `step` inside the `tick` function, so no change, but also adding initial value during the construction of the class for the sake of correctness.]

PiperOrigin-RevId: 411637500

google-research/scenic