Basic conv model with L2 modality.

def next_frame_l2():
  """Basic conv model with L2 modality."""
  hparams = next_frame_basic_deterministic()
  hparams.loss["targets"] = modalities.video_l2_loss
  hparams.top["targets"] = modalities.video_l1_top
  hparams.video_modality_loss_cutoff = 2.4
  return hparams