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: Task name as string.
docstr_tok: [":","return",":","Task","name","as","string","."]
code: def __str__(self):
      """:return: Task name as string."""
      return "Ranking Task"
  
imports: from matchzoo.engine import base_task
label_desc: Use image node_id 5 for calling the Ranking obj's underlying member method code with example usage: obj.__str__() and returns: str