code: def _get_execute_combine_result(
      cls, op: DataFrameGroupByAgg, in_data: pd.DataFrame
  ) -> Union[pd.DataFrame, pd.Series]:
      # in_data.index.names means MultiIndex (groupby on multi cols)
      index_col = in_data.index.name or in_data.index.names
      res = in_data.reset_index().drop_duplicates().set_index(index_col)
      if op.output_types[0] == OutputType.series:
          res = res.squeeze()
      return res
  
file_path: mars/mars/dataframe/groupby/nunique.py
imports: from typing import List, Union
  import pandas
  from ...core import OutputType
  from ...utils import implements
  from .aggregation import DataFrameGroupByAgg
  from .custom_aggregation import DataFrameCustomGroupByAggMixin, register_custom_groupby_agg_func
label_desc: Use image node_id 4 for calling the DataFrameCustomGroupByNuniqueMixin obj's underlying member method code with example usage: obj._get_execute_combine_result(cls, op, in_data) and returns: res