class_docstr: Ranking Task.
  
  Examples:
      >>> ranking_task = Ranking()
      >>> ranking_task.metrics = ['map', 'ndcg']
      >>> ranking_task.output_shape
      (1,)
      >>> ranking_task.output_dtype
      <class 'float'>
      >>> print(ranking_task)
      Ranking Task
docstr: :return: target data type, expect `float` as output.
docstr_tok: [":","return",":","target","data","type",",","expect","`","float","`","as","output","."]
code: def output_dtype(self):
      """:return: target data type, expect `float` as output."""
      return float
  
imports: from matchzoo.engine import base_task
label_desc: Use image node_id 4 for calling the Ranking obj's underlying member method code with example usage: obj.output_dtype() and returns: float