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.
code: def __call__(self, *args, **params):
      # Get requested backends
      config = params.pop("config", {})
      util.config.param.update(**config)
      imports = [
          (arg, self._backends[arg])
          for arg in args
          if arg in self._backends
      ]
      for p, _val in sorted(params.items()):
          if p in self._backends:
              imports.append((p, self._backends[p]))
      if not imports:
          args = ["matplotlib"]
          imports = [("matplotlib", "mpl")]
  
      args = list(args)
      selected_backend = None
      for backend, imp in imports:
          try:
              __import__(backend)
          except ImportError:
              self.param.warning(
                  f"{backend} could not be imported, ensure {backend} is installed."
              )
          try:
              __import__(f"holoviews.plotting.{imp}")
              if selected_backend is None:
                  selected_backend = backend
          except util.VersionError as e:
              self.param.warning(
                  f"HoloViews {backend} extension could not be loaded. "
                  f"The installed {backend} version {e.version} is less than "
                  f"the required version {e.min_version}."
              )
          except Exception as e:
              self.param.warning(
                  f"Holoviews {backend} extension could not be imported, "
                  f"it raised the following exception: {type(e).__name__}('{e}')"
              )
          finally:
              Store.output_settings.allowed["backend"] = list_backends()
              Store.output_settings.allowed["fig"] = list_formats(
                  "fig", backend
              )
              Store.output_settings.allowed["holomap"] = list_formats(
                  "holomap", backend
              )
          for hook in self._backend_hooks[backend]:
              try:
                  hook()
              except Exception as e:
                  self.param.warning(
                      f"{backend} backend hook {hook} failed with "
                      f"following exception: {e}"
                  )
  
      if selected_backend is None:
          raise ImportError("None of the backends could be imported")
      Store.set_current_backend(selected_backend)
  
      import panel as pn
  
      if (
          params.get("enable_mathjax", False)
          and selected_backend == "bokeh"
      ):
          pn.extension("mathjax")
  
      if pn.config.comms == "default":
          if "google.colab" in sys.modules:
              pn.config.comms = "colab"
              return
  
          if "VSCODE_CWD" in os.environ or "VSCODE_PID" in os.environ:
              pn.config.comms = "vscode"
              self._ignore_bokeh_warnings()
              return
  
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 1 for calling the extension obj's underlying member method code with example usage: obj.__call__() without return types