class_docstr: Helper utility used to load holoviews extensions. These can be
  plotting extensions, element extensions or anything else that can be
  registered to work with HoloViews.
docstr: Registers a hook which is run when a backend is loaded
docstr_tok: ["Registers","a","hook","which","is","run","when","a","backend","is","loaded"]
code: def register_backend_callback(cls, backend, callback):
      """Registers a hook which is run when a backend is loaded"""
      cls._backend_hooks[backend].append(callback)
  
imports: import inspect
  import os
  import shutil
  import sys
  from collections import defaultdict
  from inspect import Parameter, Signature
  from pathlib import Path
  from types import FunctionType
  import param
  from pyviz_comms import extension
  from ..core import Dataset, Dimensioned, DynamicMap, HoloMap, Store, StoreOptions, ViewableElement, util
  from ..core.operation import Operation, OperationCallable
  from ..core.options import Keywords, Options, options_policy
  from ..core.overlay import Overlay
  from ..core.util import merge_options_to_dict
  from ..operation.element import function
  from ..streams import Params, Stream, streams_list_from_dict
  from .settings import OutputSettings, list_backends, list_formats
label_desc: Use image node_id 2 for calling the extension obj's underlying member method code with example usage: obj.register_backend_callback(cls, backend, callback) without return types