Set of hyperparameters.

def transformer_symshard_base():
  """Set of hyperparameters."""
  hparams = common_hparams.basic_params1()
  hparams.hidden_size = 256
  hparams.batch_size = 2048
  hparams.max_length = 0
  # All hyperparameters ending in "dropout" are automatically set to 0.0
  # when not in training mode.
  hparams.layer_prepostprocess_dropout = 0.2
  hparams.add_hparam("attention_dropout", 0.1)
  hparams.add_hparam("relu_dropout", 0.0)
  hparams.add_hparam("relu_dropout_broadcast_dims", "1")
  hparams.layer_prepostprocess_dropout = 0.1
  hparams.layer_prepostprocess_dropout_broadcast_dims = "1"  # length
  hparams.label_smoothing = 0.1
  hparams.clip_grad_norm = 0.  # i.e. no gradient clipping
  hparams.optimizer = "Adafactor"
  hparams.learning_rate_schedule = "rsqrt_decay"
  hparams.learning_rate_warmup_steps = 10000
  hparams.initializer_gain = 1.0
  hparams.initializer = "uniform_unit_scaling"
  hparams.weight_decay = 0.0
  # TODO(noam): use this to control sharing.  We now share always
  hparams.shared_embedding_and_softmax_weights = True
  # we only want one data shard.
  hparams.no_data_parallelism = True
  # bypass the symbol modality so that we can use model parallelism.
  hparams.bottom = {
      "inputs": modalities.identity_bottom,
      "targets": modalities.identity_bottom,
  }
  hparams.top = {
      "targets": modalities.identity_top,
  }
  hparams.add_hparam("filter_size", 1280)
  hparams.add_hparam("mix_fraction", 0.5)
  # attention-related flags
  hparams.add_hparam("multihead_attention_num_heads", 4)
  hparams.add_hparam("multihead_attention_key_channels", 0)
  hparams.add_hparam("multihead_attention_value_channels", 0)
  hparams.add_hparam("pos", "timing")  # timing, none
  hparams.add_hparam(
      "encoder_layers", ("n,att,m,d,a," "n,ffn,m,d,a,") * 6 + "n,d")
  hparams.add_hparam(
      "decoder_layers",
      ("n,att,m,d,a," "n,enc-att,m,d,a," "n,ffn,m,d,a,") * 6 + "n,d")
  # Number of model shards - each one has separate parameters.
  # Changing this number invalidates checkpoints.
  hparams.add_hparam("num_model_shards", 8)
  return hparams