class_docstr: Singleton metaclass for :class:`fiftyone.core.frame.Frame`.
  
  This metaclass maintains a weakref dictionary of all in-memory
  :class:`fiftyone.core.frame.Frame` instances keyed by
  ``[collection name][sample ID][frame number]``.
docstr: Returns a frame number -> Frame dict containing all in-memory frame
  instances for the specified sample.
docstr_tok: ["Returns","a","frame","number","-",">","Frame","dict","containing","all","in-memory","frame","instances","for","the","specified","sample","."]
code: def _get_instances(cls, collection_name, sample_id):
      """Returns a frame number -> Frame dict containing all in-memory frame
      instances for the specified sample.
      """
      return dict(
          cls._instances.get(collection_name, {}).get(
              str(sample_id), {}
          )
      )
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 5 for calling the FrameSingleton obj's underlying member method code with example usage: obj._get_instances(cls, collection_name, sample_id) and returns: dict