Pong base parameters.

def ppo_atari_base():
  """Pong base parameters."""
  hparams = ppo_discrete_action_base()
  hparams.learning_rate_constant = 1e-4
  hparams.epoch_length = 200
  hparams.gae_gamma = 0.985
  hparams.gae_lambda = 0.985
  hparams.entropy_loss_coef = 0.003
  hparams.value_loss_coef = 1
  hparams.optimization_epochs = 3
  hparams.epochs_num = 1000
  hparams.policy_network = "feed_forward_cnn_small_categorical_policy"
  hparams.clipping_coef = 0.2
  hparams.optimization_batch_size = 20
  hparams.clip_grad_norm = 0.5
  return hparams