def ReZeroTransformerEncoder(vocab_size,
                             n_classes=10,
                             d_model=512,
                             d_ff=2048,
                             n_layers=6,
                             n_heads=8,
                             dropout=0.1,
                             dropout_shared_axes=None,
                             max_len=2048,
                             mode='train',
                             ff_activation=tl.Relu):
  """Returns a ReZero transformer encoder model.

  The input to the model is a tensor of tokens.

  Args:
    vocab_size: int: vocab size
    n_classes: how many classes on output
    d_model: int:  depth of embedding
    d_ff: int: depth of feed-forward layer
    n_layers: int: number of encoder/decoder layers
    n_heads: int: number of attention heads
    dropout: float: dropout rate (how much to drop out)
    dropout_shared_axes: axes on which to share dropout mask
    max_len: int: maximum symbol length for positional encoding
    mode: str: 'train' or 'eval'
    ff_activation: the non-linearity in feed-forward layer

  Returns:
    A ReZero transformer model as a layer that maps from a tensor of tokens to
    activations over a set of output classes.
  """
  positional_encoder = [
      tl.Embedding(vocab_size, d_model),
      tl.Dropout(rate=dropout, shared_axes=dropout_shared_axes, mode=mode),
      tl.PositionalEncoding(max_len=max_len)]

  encoder_blocks = [
      _EncoderBlock(d_model, d_ff, n_heads, dropout, dropout_shared_axes,
                    mode, ff_activation)
      for i in range(n_layers)]

  # Assemble and return the model.
  return tl.Serial(                               # toks
      # Encode.
      tl.Branch(
          positional_encoder, tl.PaddingMask()),  # vecs masks
      encoder_blocks,                             # vecs masks
      tl.Select([0], n_in=2),                     # vecs
      tl.LayerNorm(),                             # vecs

      # Map to output categories.
      tl.Mean(axis=1),                            # vecs
      tl.Dense(n_classes),                        # vecs
  )
Returns a ReZero transformer encoder model.

  The input to the model is a tensor of tokens.

  Args:
    vocab_size: int: vocab size
    n_classes: how many classes on output
    d_model: int:  depth of embedding
    d_ff: int: depth of feed-forward layer
    n_layers: int: number of encoder/decoder layers
    n_heads: int: number of attention heads
    dropout: float: dropout rate (how much to drop out)
    dropout_shared_axes: axes on which to share dropout mask
    max_len: int: maximum symbol length for positional encoding
    mode: str: 'train' or 'eval'
    ff_activation: the non-linearity in feed-forward layer

  Returns:
    A ReZero transformer model as a layer that maps from a tensor of tokens to
    activations over a set of output classes.
  
trax/models/research/rezero.py