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: a list of available metrics.
docstr_tok: [":","return",":","a","list","of","available","metrics","."]
code: def list_available_metrics(cls) -> list:
      """:return: a list of available metrics."""
      return ["map"]
  
imports: from matchzoo.engine import base_task
label_desc: Use image node_id 2 for calling the Ranking obj's underlying member method code with example usage: obj.list_available_metrics(cls) and returns: list