Merge pull request #7602 from guoshengCS/add-dot_product_attention

Add Python wrapper for dot-product-attention.

doc/api/v2/fluid/layers.rst

@@ -364,6 +364,12 @@ split
 ..  autofunction:: paddle.v2.fluid.layers.split
     :noindex:
 
+
+matmul
+------
+..  autofunction:: paddle.v2.fluid.layers.matmul
+    :noindex:
+
 logsigmoid
 ----------
 ..  autofunction:: paddle.v2.fluid.layers.logsigmoid

doc/api/v2/fluid/nets.rst

@@ -25,3 +25,9 @@ glu
 ..  autofunction:: paddle.v2.fluid.nets.glu
     :noindex:
 
+
+dot_product_attention
+---------------------
+..  autofunction:: paddle.v2.fluid.nets.dot_product_attention
+    :noindex:
+

python/paddle/v2/fluid/layers/nn.py

@@ -50,6 +50,7 @@ __all__ = [
     'sequence_last_step',
     'dropout',
     'split',
+    'matmul',
 ]
 
 
@@ -1597,3 +1598,73 @@ def split(input, num_or_sections, dim=-1):
             'axis': dim
         })
     return outs
+
+
+def matmul(x, y, transpose_x=False, transpose_y=False, name=None):
+    """
+    Applies matrix multipication to two tensors. Currently only rank 1 to rank 
+    3 input tensors are supported.
+
+    The actual behavior depends on the shapes of :math:`x`, :math:`y` and the 
+    flag values of :attr:`transpose_x`, :attr:`transpose_y`. Specifically:
+
+    - If a transpose flag is specified, the last two dimensions of the tensor 
+      are transposed. If the tensor is rank-1 of shape :math:`[D]`, then for 
+      :math:`x` it is treated as :math:`[1, D]` in nontransposed form and as 
+      :math:`[D, 1]` in transposed form, whereas for :math:`y` it is the 
+      opposite: It is treated as :math:`[D, 1]` in nontransposed form and as 
+      :math:`[1, D]` in transposed form.
+
+    - After transpose, the two tensors are 2-D or 3-D and matrix multipication 
+      performs in the following way.
+
+      - If both are 2-D, they are multiplied like conventional matrices.
+      - If either is 3-D, it is treated as a stack of matrices residing in the 
+        last two dimensions and a batched matrix multiply supporting broadcast 
+        applies on the two tensors.
+
+    Also note that if the raw tensor :math:`x` or :math:`y` is rank-1 and 
+    nontransposed, the prepended or appended dimension :math:`1` will be 
+    removed after matrix multipication.
+
+    Args:
+        x (Variable): The input variable which is a Tensor or LoDTensor.
+        y (Variable): The input variable which is a Tensor or LoDTensor.
+        transpose_x (bool): Whether to transpose :math:`x` before multiplication.
+        transpose_y (bool): Whether to transpose :math:`y` before multiplication.
+        name(str|None): A name for this layer(optional). If set None, the layer 
+            will be named automatically.
+
+    Returns:
+        Variable: The product Tensor variable.
+
+    Examples:
+        .. code-block:: python
+
+            # Examples to clarify shapes of the inputs and output
+            # x: [B, M, K], y: [B, K, N]
+            fluid.layers.matmul(x, y)  # out: [B, M, N]
+            # x: [B, M, K], y: [K, N]
+            fluid.layers.matmul(x, y)  # out: [B, M, N]
+            # x: [B, M, K], y: [K]
+            fluid.layers.matmul(x, y)  # out: [B, M]
+            # x: [M, K], y: [K, N]
+            fluid.layers.matmul(x, y)  # out: [M, N]
+            # x: [K], y: [K]
+            fluid.layers.matmul(x, y)  # out: [1]
+            # x: [M], y: [N]
+            fluid.layers.matmul(x, y, True, True)  # out: [M, N]
+    """
+    helper = LayerHelper('matmul', **locals())
+    assert max(
+        len(x.shape), len(y.shape)
+    ) <= 3, 'Currently only rank 1 to rank 3 input tensors are supported.'
+    out = helper.create_tmp_variable(dtype=helper.input_dtype())
+    helper.append_op(
+        type='matmul',
+        inputs={'X': x,
+                'Y': y},
+        outputs={'Out': out},
+        attrs={'transpose_X': transpose_x,
+               'transpose_Y': transpose_y})
+    return out

python/paddle/v2/fluid/nets.py

@@ -17,6 +17,7 @@ __all__ = [
     "simple_img_conv_pool",
     "sequence_conv_pool",
     "glu",
+    "dot_product_attention",
 ]
 
 
@@ -150,3 +151,55 @@ def glu(input, dim=-1):
     act_b = layers.sigmoid(x=b)
     out = layers.elementwise_mul(x=a, y=act_b)
     return out
+
+
+def dot_product_attention(querys, keys, values):
+    """
+    The dot-product attention.
+
+    Attention mechanism can be seen as mapping a query and a set of key-value 
+    pairs to an output. The output is computed as a weighted sum of the values, 
+    where the weight assigned to each value is computed by a compatibility 
+    function (dot-product here) of the query with the corresponding key.
+    
+    The dot-product attention can be implemented through (batch) matrix 
+    multipication as follows:
+
+        .. math::
+
+            Attention(Q, K, V)= softmax(QK^\mathrm{T})V
+
+    Refer to `Attention Is All You Need 
+    <https://arxiv.org/pdf/1706.03762.pdf>`_.
+
+    Note that batch data containing sequences with different lengths is not 
+    supported by this because of the (batch) matrix multipication.
+    
+    Args:
+        query (Variable): The input variable which is a Tensor or LoDTensor.
+        key (Variable): The input variable which is a Tensor or LoDTensor.
+        value (Variable): The input variable which is a Tensor or LoDTensor.
+
+    Returns:
+        tuple: The Tensor variables representing the output and attention scores.
+
+    Examples:
+        .. code-block:: python
+
+            # Suppose q, k, v are tensor variables with the following shape:
+            # q: [3, 5, 9], k: [3, 6, 9], v: [3, 6, 10]
+            out, attn_scores = fluid.nets.dot_product_attention(q, k, v)
+            out.shape  # [3, 5, 10]
+            attn_scores.shape  # [3, 5, 6]
+    """
+    assert keys.shape[-2] == values.shape[
+        -2], 'The shapes of keys and values mismatch.'
+    assert querys.shape[-1] == keys.shape[
+        -1], 'The shapes of querys and keys mismatch.'
+    product = layers.matmul(x=querys, y=keys, transpose_y=True)
+    attn_scores = layers.reshape(
+        x=layers.reshape(
+            x=product, shape=[-1, product.shape[-1]], act='softmax'),
+        shape=product.shape)
+    out = layers.matmul(attn_scores, values)
+    return out, attn_scores

python/paddle/v2/fluid/tests/test_matmul_op.py

@@ -96,18 +96,18 @@ class Generator(object):
         self.outputs = {'Out': Out}
 
     def test_check_output(self):
-        self.check_output(atol=1e-2)
+        self.check_output(atol=1e-3)
 
     def test_check_grad_normal(self):
-        self.check_grad(['X', 'Y'], 'Out', max_relative_error=0.5)
+        self.check_grad(['X', 'Y'], 'Out', max_relative_error=1e-3)
 
     def test_check_grad_ignore_x(self):
         self.check_grad(
-            ['Y'], 'Out', max_relative_error=0.5, no_grad_set=set("X"))
+            ['Y'], 'Out', max_relative_error=1e-3, no_grad_set=set("X"))
 
     def test_check_grad_ignore_y(self):
         self.check_grad(
-            ['X'], 'Out', max_relative_error=0.5, no_grad_set=set('Y'))
+            ['X'], 'Out', max_relative_error=1e-3, no_grad_set=set('Y'))
 
 
 # Generate test cases for all possibilities