Update Comet Callback documentation

[Lothiraldan]

The current Comet callback is broken. The Comet team is now maintaining it inside the Comet SDK. Update the documentation to point to the new version of the callback.

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[Lothiraldan]

I wasn't sure exactly what to do with the current Callback in fastai repository (https://github.com/fastai/fastai/blob/master/fastai/callback/comet.py). Given that it's broken when you try to instantiate the callback:

File ~/.virtualenvs/comet3.9/lib/python3.9/site-packages/fastai/callback/comet.py:24, in CometCallback.__init__(self, project_name, log_model_weights)
     22 def __init__(self, project_name, log_model_weights=True):
     23     self.log_model_weights = log_model_weights
---> 24     self.keep_experiment_running = keep_experiment_running
     25     self.project_name = project_name
     26     self.experiment = None

NameError: name 'keep_experiment_running' is not defined

I think it's safe to delete the comet module. Alternatively, we can update the current callback to show a warning pointing to the new one maintained by Comet. Please let me know what do you prefer.