def EveryOtherLayerDropTransformerLM(vocab_size,
                                     d_model=512,
                                     d_ff=2048,
                                     n_layers=6,
                                     n_heads=8,
                                     dropout=0.1,
                                     max_len=2048,
                                     mode='train',
                                     ff_activation=tl.Relu,
                                     skip_mode='even',
                                     skip_fraction=0.5,
                                     eval_skip_fraction=0.0):
  """Returns an "EveryOther" LayerDrop Transformer language model.

  During each training step it either runs all layers, or skips a subset of
  layers. This subset is the same every time, and it is specified by
  "skip_mode".
  The input to the model is a tensor of tokens. (This model uses only the
  decoder part of the overall Transformer.)

  Args:
    vocab_size: int: vocab size
    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)
    max_len: int: maximum symbol length for positional encoding
    mode: str: 'train', 'eval' or 'predict', predict mode is for fast inference
    ff_activation: the non-linearity in feed-forward layer
    skip_mode: which layers to skip when skipping: even/odd/1half/2half.
    skip_fraction: fraction of times to skip layers
    eval_skip_fraction: fraction of times to skip layers during eval

  Returns:
    A Transformer language model as a layer that maps from a tensor of tokens
    to activations over a vocab set.
  """
  embedder = [
      tl.Embedding(vocab_size, d_model),
      tl.Dropout(rate=dropout, mode=mode),
      tl.PositionalEncoding(max_len=max_len, mode=mode),
  ]

  if mode == 'train':
    pass
  else:
    skip_fraction = eval_skip_fraction

  skip_mode_funs = {  # which layers should be skipped?
      'even': (lambda num: num%2 == 0),  # 0th layer is even
      'odd': (lambda num: num%2 == 1),
      '1half': (lambda num: num < (n_layers/2)),
      '2half': (lambda num: num >= (n_layers/2)),
  }

  skip_mode_fun = skip_mode_funs[skip_mode]

  @assert_shape('...sd,->...sd,')
  def ConditionedBlock(current_layer_num):
    return tl.Serial(
        # stack: embedding, n_layers_to_keep
        tl.Select([1, 0, 1]),  # n_layers_to_keep, embedding, n_layers_to_keep
        tl.Cond(
            # if random() > skip_fraction OR layer not in skip_mode ...
            LargerThan(skip_fraction if skip_mode_fun(current_layer_num)
                       else 0.0),
            # then: run block
            tl.Serial(transformer._DecoderBlock(  # pylint: disable=g-complex-comprehension,protected-access
                d_model, d_ff, n_heads, dropout, [], mode, ff_activation))
            # else: noop (implicit)
            )
        # stack: embedding, n_layers_to_keep
        )

  return tl.Serial(
      tl.ShiftRight(mode=mode),
      embedder,
      # stack: embedding
      tl.RandomUniform(0., 1., sync=True),
      # stack: n_layers_to_keep, embedding
      tl.Swap(),
      # stack: embedding, n_layers_to_keep
      [ConditionedBlock(i) for i in range(n_layers)],
      # stack: embedding, n_layers_to_keep
      tl.Select([0], n_in=2),  # stack: embedding
      tl.LayerNorm(),
      tl.Dense(vocab_size),
  )
Returns an "EveryOther" LayerDrop Transformer language model.

  During each training step it either runs all layers, or skips a subset of
  layers. This subset is the same every time, and it is specified by
  "skip_mode".
  The input to the model is a tensor of tokens. (This model uses only the
  decoder part of the overall Transformer.)

  Args:
    vocab_size: int: vocab size
    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)
    max_len: int: maximum symbol length for positional encoding
    mode: str: 'train', 'eval' or 'predict', predict mode is for fast inference
    ff_activation: the non-linearity in feed-forward layer
    skip_mode: which layers to skip when skipping: even/odd/1half/2half.
    skip_fraction: fraction of times to skip layers
    eval_skip_fraction: fraction of times to skip layers during eval

  Returns:
    A Transformer language model as a layer that maps from a tensor of tokens
    to activations over a vocab set.
  
trax/models/research/layerdrop_transformer.py