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: output shape of a single sample of the task.
docstr_tok: [":","return",":","output","shape","of","a","single","sample","of","the","task","."]
code: def output_shape(self) -> tuple:
      """:return: output shape of a single sample of the task."""
      return (1,)
  
imports: from matchzoo.engine import base_task
label_desc: Use image node_id 3 for calling the Ranking obj's underlying member method code with example usage: obj.output_shape() and returns: int