code: def _default_gradient(self):
      return None
  
imports: import ipywidgets
  from traitlets import Float, Bool, Unicode, HasTraits, default, List, observe
  from .None import bounds
  from .locations import locations_docstring
  from .None import geotraitlets
  from .maps import GMapsWidgetMixin
  from ._docutils import doc_subst
label_desc: Use image node_id 1 for calling the _HeatmapOptionsMixin obj's underlying member method code with example usage: obj._default_gradient() and returns: None