class_docstr: Groups an Element pair such that we have (ElementX, List[ElementY]).
code: def get_values(self, key: ElementX) -> List[ElementY]:  # noqa: D
      if key not in self._groups:
          raise KeyError(f"Unable to find `{key}` in ElementGrouper")
      return self._groups[key]
  
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 3 for calling the ElementGrouper obj's underlying member method code with example usage: obj.get_values(key) and returns: self