comet ml integration - add option to continue training
### Feature request

when continuing training, it would be great to be able to also continue to log the training metrics to the comet ml experiment. comet_ml has a feature for it : it uses the class ExistingExperiment ( see https://www.comet.ml/docs/python-sdk/ExistingExperiment/ )
Thanks

### Motivation

It would be much more clear and organized if I could continue to log the loss in the same graph when I continue the training of a model.

### Your contribution

No contribution
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