text: Title: IndexError: Cannot choose from an empty sequence
  Body: ### Description
  When I tried to train the a deterministic model in reinforcement learning project, I suddenly got this IndexError in 60000 steps. I didn't change any code in the T2T project. Now, I can't continue training. It shows
  
  File "/home/guest/tensor2tensor/tensor2tensor/data_generators/gym_env.py", line 186, in start_new_epoch
      self._load_epoch_data(load_data_dir)
    File "/home/guest/tensor2tensor/tensor2tensor/data_generators/gym_env.py", line 531, in _load_epoch_data
      raise ValueError("Some data is missing, the experiment might've been "
  ValueError: Some data is missing, the experiment might've been interupted during generating data.
  
  ### Environment information
  Linux version 4.18.0-18-generic (buildd@lcy01-amd64-006) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #19~18.04.1-Ubuntu SMP Fri Apr 5 10:22:13 UTC 2019
  
  $ pip freeze | grep tensor
  mesh-tensorflow==0.0.5
  tensor2tensor==1.13.1
  tensorboard==1.13.1
  tensorflow==1.13.1
  tensorflow-datasets==1.0.1
  tensorflow-estimator==1.13.0
  tensorflow-metadata==0.13.0
  tensorflow-probability==0.6.0
  
  $ python -V
  Python 3.6.8 :: Anaconda, Inc.
  
  INFO:tensorflow:Timing: 2:35:05.578154
  INFO:tensorflow:Setting T2TModel mode to 'infer'
  INFO:tensorflow:Setting hparams.dropout to 0.0
  INFO:tensorflow:Setting hparams.label_smoothing to 0.0
  INFO:tensorflow:Setting hparams.layer_prepostprocess_dropout to 0.0
  INFO:tensorflow:Setting hparams.symbol_dropout to 0.0
  INFO:tensorflow:Setting hparams.residual_dropout to 0.0
  INFO:tensorflow:Using variable initializer: uniform_unit_scaling
  INFO:tensorflow:Transforming feature 'input_action' with symbol_modality_6_64.bottom
  INFO:tensorflow:Transforming feature 'input_reward' with symbol_modality_3_64.bottom
  INFO:tensorflow:Transforming feature 'inputs' with video_modality.bottom
  INFO:tensorflow:Transforming feature 'target_action' with symbol_modality_6_64.targets_bottom
  INFO:tensorflow:Transforming feature 'target_reward' with symbol_modality_3_64.targets_bottom
  INFO:tensorflow:Transforming feature 'targets' with video_modality.targets_bottom
  INFO:tensorflow:Building model body
  INFO:tensorflow:Transforming body output with video_modality.top
  INFO:tensorflow:Transforming body output with symbol_modality_3_64.top
  INFO:tensorflow:Restoring checkpoint /home/guest/t2t_train/mb_det_pong_random/world_model/model.ckpt-60000
  INFO:tensorflow:Restoring parameters from /home/guest/t2t_train/mb_det_pong_random/world_model/model.ckpt-60000
  Traceback (most recent call last):
    File "/home/guest/miniconda3/envs/tensor2tensor/lib/python3.6/runpy.py", line 193, in _run_module_as_main
      "__main__", mod_spec)
    File "/home/guest/miniconda3/envs/tensor2tensor/lib/python3.6/runpy.py", line 85, in _run_code
      exec(code, run_globals)
    File "/home/guest/tensor2tensor/tensor2tensor/rl/trainer_model_based.py", line 389, in <module>
      tf.app.run()
    File "/home/guest/miniconda3/envs/tensor2tensor/lib/python3.6/site-packages/tensorflow/python/platform/app.py", line 125, in run
      _sys.exit(main(argv))
    File "/home/guest/tensor2tensor/tensor2tensor/rl/trainer_model_based.py", line 384, in main
      training_loop(hp, FLAGS.output_dir)
    File "/home/guest/tensor2tensor/tensor2tensor/rl/trainer_model_based.py", line 356, in training_loop
      env, hparams, directories["world_model"], debug_video_path
    File "/home/guest/tensor2tensor/tensor2tensor/rl/rl_utils.py", line 158, in evaluate_world_model
      subsequence_length + frame_stack_size
    File "/home/guest/tensor2tensor/tensor2tensor/rl/rl_utils.py", line 336, in random_rollout_subsequences
      return [choose_subsequence() for _ in range(num_subsequences)]
    File "/home/guest/tensor2tensor/tensor2tensor/rl/rl_utils.py", line 336, in <listcomp>
      return [choose_subsequence() for _ in range(num_subsequences)]
    File "/home/guest/tensor2tensor/tensor2tensor/rl/rl_utils.py", line 328, in choose_subsequence
      rollout = random.choice(rollouts)
    File "/home/guest/miniconda3/envs/tensor2tensor/lib/python3.6/random.py", line 260, in choice
      raise IndexError('Cannot choose from an empty sequence') from None
  IndexError: Cannot choose from an empty sequence
  
  