Update CometMLTracker to allow re-using experiment (#3328)
* Update CometMLTracker to allow re-using experiment
Update CometMLTracker to use new `comet_ml.start` function to create
Experiments, this way end-users can create online, offline experiments, append
data to an existing experiment and it also automatically re-use a running
experiment if one is present rather than creating a new one.
* Add back calling Experiment.end in finish
As `accelerator.end_training` is supposed to be called at the very end of
training by the user, users will still be able to log data after the main
training loop and this is needed for Offline Experiment to create the offline
archive.
* Update CometTracker behavior based on the version of the package
Use new method only for recent version of comet_ml
