code: def __call__(self, a):
      if not isinstance(a, ENTITY_TYPE):
          a = asseries(a)
      self.output_types = [OutputType.tensor]
      return self.new_tileables(
          [a], shape=(np.nan,), dtype=a.dtype, order=TensorOrder.C_ORDER
      )[0]
  
file_path: mars/mars/dataframe/reduction/unique.py
imports: import pandas
  import numpy
  from ...None import opcodes
  from ...core import OutputType, ENTITY_TYPE
  from ...tensor.core import TensorOrder
  from ...utils import lazy_import
  from ..initializer import Series
  from .core import DataFrameReductionOperand, DataFrameReductionMixin, CustomReduction
label_desc: Use image node_id 3 for calling the DataFrameUnique obj's underlying member method code with example usage: obj.__call__(a) and returns: self