Class: Transformers::Bert::BertSelfAttention

Inherits:
Torch::NN::Module
  • Object
show all
Defined in:
lib/transformers/models/bert/modeling_bert.rb

Instance Method Summary collapse

Constructor Details

#initialize(config, position_embedding_type: nil) ⇒ BertSelfAttention

Returns a new instance of BertSelfAttention.



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# File 'lib/transformers/models/bert/modeling_bert.rb', line 80

def initialize(config, position_embedding_type: nil)
  super()
  if config.hidden_size % config.num_attention_heads != 0 && !config.embedding_size
    raise ArgumentError,
      "The hidden size (#{config.hidden_size}) is not a multiple of the number of attention " +
      "heads (#{config.num_attention_heads})"
  end

  @num_attention_heads = config.num_attention_heads
  @attention_head_size = (config.hidden_size / config.num_attention_heads).to_i
  @all_head_size = @num_attention_heads * @attention_head_size

  @query = Torch::NN::Linear.new(config.hidden_size, @all_head_size)
  @key = Torch::NN::Linear.new(config.hidden_size, @all_head_size)
  @value = Torch::NN::Linear.new(config.hidden_size, @all_head_size)

  @dropout = Torch::NN::Dropout.new(p: config.attention_probs_dropout_prob)
  @position_embedding_type = position_embedding_type || config.position_embedding_type || "absolute"
  if @position_embedding_type == "relative_key" || @position_embedding_type == "relative_key_query"
    @max_position_embeddings = config.max_position_embeddings
    @distance_embedding = Torch::NN::Embedding.new(2 * config.max_position_embeddings - 1, @attention_head_size)
  end

  @is_decoder = config.is_decoder
end

Instance Method Details

#forward(hidden_states, attention_mask: nil, head_mask: nil, encoder_hidden_states: nil, encoder_attention_mask: nil, past_key_value: nil, output_attentions: false) ⇒ Object 



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# File 'lib/transformers/models/bert/modeling_bert.rb', line 112

def forward(
  hidden_states,
  attention_mask: nil,
  head_mask: nil,
  encoder_hidden_states: nil,
  encoder_attention_mask: nil,
  past_key_value: nil,
  output_attentions: false
)
  mixed_query_layer = @query.(hidden_states)

  # If this is instantiated as a cross-attention module, the keys
  # and values come from an encoder; the attention mask needs to be
  # such that the encoder's padding tokens are not attended to.
  is_cross_attention = !encoder_hidden_states.nil?

  if is_cross_attention && !past_key_value.nil?
    # reuse k,v, cross_attentions
    key_layer = past_key_value[0]
    value_layer = past_key_value[1]
    attention_mask = encoder_attention_mask
  elsif is_cross_attention
    key_layer = transpose_for_scores(@key.(encoder_hidden_states))
    value_layer = transpose_for_scores(@value.(encoder_hidden_states))
    attention_mask = encoder_attention_mask
  elsif !past_key_value.nil?
    key_layer = transpose_for_scores(@key.(hidden_states))
    value_layer = transpose_for_scores(@value.(hidden_states))
    key_layer = Torch.cat([past_key_value[0], key_layer], dim: 2)
    value_layer = Torch.cat([past_key_value[1], value_layer], dim: 2)
  else
    key_layer = transpose_for_scores(@key.(hidden_states))
    value_layer = transpose_for_scores(@value.(hidden_states))
  end

  query_layer = transpose_for_scores(mixed_query_layer)

  _use_cache = !past_key_value.nil?
  if @is_decoder
    # if cross_attention save Tuple(torch.Tensor, torch.Tensor) of all cross attention key/value_states.
    # Further calls to cross_attention layer can then reuse all cross-attention
    # key/value_states (first "if" case)
    # if uni-directional self-attention (decoder) save Tuple(torch.Tensor, torch.Tensor) of
    # all previous decoder key/value_states. Further calls to uni-directional self-attention
    # can concat previous decoder key/value_states to current projected key/value_states (third "elif" case)
    # if encoder bi-directional self-attention `past_key_value` is always `None`
    past_key_value = [key_layer, value_layer]
  end

  # Take the dot product between "query" and "key" to get the raw attention scores.
  attention_scores = Torch.matmul(query_layer, key_layer.transpose(-1, -2))

  if @position_embedding_type == "relative_key" || @position_embedding_type == "relative_key_query"
    raise Todo
  end

  attention_scores = attention_scores / Math.sqrt(@attention_head_size)
  if !attention_mask.nil?
    # Apply the attention mask is (precomputed for all layers in BertModel forward() function)
    attention_scores = attention_scores + attention_mask
  end

  # Normalize the attention scores to probabilities.
  attention_probs = Torch::NN::Functional.softmax(attention_scores, dim: -1)

  # This is actually dropping out entire tokens to attend to, which might
  # seem a bit unusual, but is taken from the original Transformer paper.
  attention_probs = @dropout.(attention_probs)

  # Mask heads if we want to
  if !head_mask.nil?
    attention_probs = attention_probs * head_mask
  end

  context_layer = Torch.matmul(attention_probs, value_layer)

  context_layer = context_layer.permute(0, 2, 1, 3).contiguous
  new_context_layer_shape = context_layer.size[...-2] + [@all_head_size]
  context_layer = context_layer.view(new_context_layer_shape)

  outputs = output_attentions ? [context_layer, attention_probs] : [context_layer]

  if @is_decoder
    outputs = outputs + [past_key_value]
  end
  outputs
end

#transpose_for_scores(x) ⇒ Object 



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# File 'lib/transformers/models/bert/modeling_bert.rb', line 106

def transpose_for_scores(x)
  new_x_shape = x.size[...-1] + [@num_attention_heads, @attention_head_size]
  x = x.view(new_x_shape)
  x.permute(0, 2, 1, 3)
end