class_docstr: Disable the ``{before,after}_epoch``, ``{before,after}_run``,
  ``trigger_{epoch,step}``
  methods of a callback, unless some condition satisfies.
  The other methods are unaffected.
  
  A more accurate name for this callback should be "DisableCallbackUnless", but that's too ugly.
  
  Note:
      If you use ``{before,after}_run``,
      ``pred`` will be evaluated only in ``before_run``.
code: def _trigger_step(self):
      if self._pred(self):
          super(EnableCallbackIf, self)._trigger_step()
  
imports: from .base import Callback, ProxyCallback
label_desc: Use image node_id 7 for calling the EnableCallbackIf obj's underlying member method code with example usage: obj._trigger_step() without return types