docstr: Get the fit-compute function of the step.
  
  You can use this only when the step has been called exactly once.
  
  Returns
  -------
  Callable
  
  Raises
  ------
  AttributeError
      If the step has not been called yet or it is a shared step
      (called several times).
code: def fit_compute_func(self) -> Optional[Callable]:
      """Get the fit-compute function of the step.
  
      You can use this only when the step has been called exactly once.
  
      Returns
      -------
      Callable
  
      Raises
      ------
      AttributeError
          If the step has not been called yet or it is a shared step
          (called several times).
      """
      return self._get_step_attr("fit_compute_func")
  
imports: import inspect
  import warnings
  from typing import Any, Callable, Dict, List, Optional, Union, TYPE_CHECKING
  from baikal._core.data_placeholder import DataPlaceholder, is_data_placeholder_list
  from baikal._core.utils import listify, make_name, make_repr
label_desc: Use image node_id 15 for calling the _StepBase obj's underlying member method code with example usage: obj.fit_compute_func() and returns: self