docs: def _get_vars_to_collections(variables):
    """Returns a dict mapping variables to the collections they appear in."""
    var_to_collections = collections.defaultdict(lambda: [])
    if isinstance(variables, dict):
      variables = list(v for _, v in variable_map_items(variables))
    for graph in set(v.graph for v in variables):
      for collection_name in list(graph.collections):
        entries = set(entry for entry in graph.get_collection(collection_name)
                      if isinstance(entry, tf.Variable))
        # For legacy reasons, tf.GraphKeys.GLOBAL_VARIABLES == "variables".
        # Correcting for this here, to avoid confusion.
        if collection_name == tf.GraphKeys.GLOBAL_VARIABLES:
          collection_name = "global_variables"
        for var in entries.intersection(variables):
          var_to_collections[var].append(collection_name)
    return var_to_collections