code: def register_forward_hook(self, hook):
          r
          handle = HookHandle()
          handle.attach(self._forward_hooks, hook)
          return handle
docs: r"""Registers a forward hook on the block.
  
          The hook function is called immediately after :func:`forward`.
          It should not modify the input or output.
  
          Parameters
          ----------
          hook : callable
              The forward hook function of form `hook(block, input, output) -> None`.
  
          Returns
          -------
          :class:`mxnet.gluon.utils.HookHandle`