code: def nunique_series(series, dropna=True, combine_size=None):
      """
      Return number of unique elements in the object.
  
      Excludes NA values by default.
  
      Parameters
      ----------
      dropna : bool, default True
          Don't include NaN in the count.
      combine_size : int, optional
          The number of chunks to combine.
  
      Returns
      -------
      int
  
      See Also
      --------
      DataFrame.nunique: Method nunique for DataFrame.
      Series.count: Count non-NA/null observations in the Series.
  
      Examples
      --------
      >>> import mars.dataframe as md
      >>> s = md.Series([1, 3, 5, 7, 7])
      >>> s.execute()
      0    1
      1    3
      2    5
      3    7
      4    7
      dtype: int64
  
      >>> s.nunique().execute()
      4
      """
      op = DataFrameNunique(
          dropna=dropna,
          combine_size=combine_size,
          output_types=[OutputType.scalar],
          use_arrow_dtype=options.dataframe.use_arrow_dtype,
      )
      return op(series)
  
imports: import pandas
  from ...None import opcodes
  from ...core import OutputType
  from ...config import options
  from ...serialization.serializables import BoolField
  from ...utils import lazy_import
  from ..arrays import ArrowListArray, ArrowListDtype
  from .core import DataFrameReductionOperand, DataFrameReductionMixin, CustomReduction
label_desc: Use image node_id 2 for calling a global function with example usage: nunique_series(series, dropna, combine_size) and returns: op