code: def transformer_decoder_ffn_unit(x, hparams, nonpadding_mask=None):
      """Applies a feed-forward function which is parametrised for decoding.
  
      Args:
        x: input
        hparams: model hyper-parameters
        nonpadding_mask: optional Tensor with shape [batch_size, encoder_length]
        indicating what positions are not padding.  This is used
        to mask out padding in convoltutional layers.  We generally only
        need this mask for "packed" datasets, because for ordinary datasets,
        no padding is ever followed by nonpadding.
  
      Returns:
        the output tensor
  
      """
  
      with tf.variable_scope("ffn"):
          if hparams.transformer_ffn_type == "fc":
              y = transformer.transformer_ffn_layer(
                  common_layers.layer_preprocess(x, hparams),
                  hparams,
                  conv_padding="LEFT",
                  nonpadding_mask=nonpadding_mask,
              )
  
          if hparams.transformer_ffn_type == "sepconv":
              y = common_layers.sepconv_relu_sepconv(
                  common_layers.layer_preprocess(x, hparams),
                  filter_size=hparams.filter_size,
                  output_size=hparams.hidden_size,
                  first_kernel_size=(3, 1),
                  second_kernel_size=(5, 1),
                  padding="LEFT",
                  nonpadding_mask=nonpadding_mask,
                  dropout=hparams.relu_dropout,
              )
  
          x = common_layers.layer_postprocess(x, y, hparams)
  
      return x
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import copy
  import functools
  from six.moves import range
  from tensor2tensor.layers import common_attention
  from tensor2tensor.layers import common_layers
  from tensor2tensor.models import transformer
  from tensor2tensor.utils import contrib
  from tensor2tensor.utils import expert_utils
  import tensorflow.compat.v1
label_desc: Use image node_id 7 for calling a global function with example usage: transformer_decoder_ffn_unit(x, hparams, nonpadding_mask) and returns: x