class_docstr: Groups an Element pair such that we have (ElementX, List[ElementY]).
code: def add_value(
      self, key: ElementX, value: ElementY
  ) -> None:  # noqa: D
      self._groups[key].append(value)
  
file_path: metricflow/metricflow/model/semantics/element_group.py
imports: from __future__ import annotations
  import collections
  from typing import Dict, Generic, List, TypeVar
label_desc: Use image node_id 2 for calling the ElementGrouper obj's underlying member method code with example usage: obj.add_value(key, value) without return types