Variable getter/setter

[bedapisl]

Migrated from https://github.com/Cognexa/cxflow/issues/186

Original post:
Some variables, such as learning rate, are scalar, i.e. their shape doesn't depend on batch size. These variables can't be stated in the model outputs.

AddScalar hook should overcome this problem and add specific scalars to
epoch_data['train'][variable_name]

Such hook will be used in the beginning of the hook list.

Adam
This is actually back-end specific. We would need to enforce model API similar to this:

   @abstractmethod
   def get_variable_value(variable_name: str) -> Union[float, int, bool, str]:
      pass

   @abstractmethod
   def set_variable_value(variable_name: str, value: Union[float, int, bool, str]) -> None:
      pass

Which is actually quite interesting. We would be able to write more back-end agnostic hooks such as DecayLR etc.