Experiment `_create_object` should not be in experiment class about fedbiomed HOT 1 OPEN

I think the implementation of _create_object can be less complex if we delegate the part of passing training_plan as parameter to methods such as Aggregator.load or Strategy_load.

I don't know if we can do better, because the original attempt was to make possible to load Strategy and Aggregator with custom modules, that could be located outside aggregator or strategies folders. But we may want to remove this feature if we consider it not too relevant

from fedbiomed.