Pause and restart hyperparameter tuning.

[WilliamGracie-Langrick]

When using optuna (on its own) it is possible to "store" the current study and restart or do more trainings on it. This is done currently by storing the study on an SQL "server" which you give optuna a URL to. This should be possible currently with OptimizerOptuna, but it appears to be very difficult to achieve as the actual study is a couple of layers deep. What would useful would be to be able to store the study in the artifacts of the ClearML experiment and then be able to reference that experiment and continue that stored study from where it left off. Failing that, it would be great if the optuna functionalities could be made more accessible so we could use them more easily. Thanks in advance. I have discussed this with Martin B. on the Slack Community.