Lookup and return decorated function or class in the collection.

Lookup decorated function or class in registered_collection, in a hierarchical order. For example, when reg_key="my_model/my_exp/my_config_0", this function will return registered_collection["my_model"]["my_exp"]["my_config_0"].

registered_collection a dictionary. The decorated function or class will be retrieved from this collection.
reg_key The key for retrieving the registered function or class. If reg_key is a string, it can be hierarchical like my_model/my_exp/my_config_0

The registered function or class.

LookupError when reg_key cannot be found.