class_docstr: Mock CombineFn for test.
class_docstr_tok: ["Mock","CombineFn","for","test","."]
code: def add_input(self, accumulator, x):
      return self.add_inputs(accumulator, [x])
  
file_path: model-analysis/tensorflow_model_analysis/experimental/lazytf/core_test.py
imports: from absl.testing import absltest
  import apache_beam
  from apache_beam.testing import util
  from tensorflow_model_analysis.experimental.lazytf import core
label_desc: Use image node_id 1 for calling the _SumCombineFn obj's underlying member method code with example usage: obj.add_input(accumulator, x) and returns: self