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 _after_run(self, ctx, rv):
      if self._enabled:
          super(EnableCallbackIf, self)._after_run(ctx, rv)
  
imports: from .base import Callback, ProxyCallback
label_desc: Use image node_id 3 for calling the EnableCallbackIf obj's underlying member method code with example usage: obj._after_run(ctx, rv) without return types