class_docstr: API for policy learners.
code: def evaluate(self, env_fn, hparams, sampling_temp):
      raise NotImplementedError()
  
file_path: tensor2tensor/tensor2tensor/rl/policy_learner.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
label_desc: Use image node_id 3 for calling the PolicyLearner obj's underlying member method code with example usage: obj.evaluate(env_fn, hparams, sampling_temp) without return types