docstr: Call the custom child step function implementation.
  
  Parameters
  ----------
  v
      Nested variables to update.
  grads
      Nested gradients to update.
  ignore_missing
      Whether to ignore keys missing from the gradients which exist in
      the variables.
      Default is ``False``
docstr_tok: ["Call","the","custom","child","step","function","implementation",".","Parameters","--","--","--","--","--","v","Nested","variables","to","update",".","grads","Nested","gradients","to","update",".","ignore_missing","Whether","to","ignore","keys","missing","from","the","gradients","which","exist","in","the","variables",".","Default","is","``","False","``"]
code: def _step_fn(
      self,
      v: ivy.Container,
      grads: ivy.Container,
      ignore_missing: bool = False,
  ):
      """Call the custom child step function implementation.
  
      Parameters
      ----------
      v
          Nested variables to update.
      grads
          Nested gradients to update.
      ignore_missing
          Whether to ignore keys missing from the gradients which exist in
          the variables.
          Default is ``False``
      """
      if ignore_missing:
          return v.cont_set_at_keys(
              self._step(v.cont_at_key_chains(grads), grads)
          )
      return self._step(v, grads)
  
imports: import abc
  from typing import Union, Optional, Callable
  import ivy
label_desc: Use image node_id 3 for calling the Optimizer obj's underlying member method code with example usage: obj._step_fn(v, grads, ignore_missing) and returns: self, v