Added Support for Returning Attention Scores in TransformerEncoder ca…

…ll (keras-team#1879)

* Added: Return attention scores argument to transformer encoder

* Added: docstring for return_attention_scores and added a test to chek the working of the argument

* Fixed: Test case by removing print stmts and using self.assertAllEqual

* Fixed: Linting

keras_hub/src/layers/modeling/transformer_encoder.py

@@ -170,7 +170,12 @@ def build(self, inputs_shape):
         self.built = True
 
     def call(
-        self, inputs, padding_mask=None, attention_mask=None, training=None
+        self,
+        inputs,
+        padding_mask=None,
+        attention_mask=None,
+        training=None,
+        return_attention_scores=False,
     ):
         """Forward pass of the TransformerEncoder.
 
@@ -185,6 +190,7 @@ def call(
                 [batch_size, sequence_length, sequence_length].
             training: a boolean indicating whether the layer should behave in
                 training mode or in inference mode.
+            return_attention_scores: a boolean indicating whether the output should be `(attention_output, attention_scores)` if `True` or `attention_output` if `False`. Defaults to `False`.
 
         Returns:
             A Tensor of the same shape as the `inputs`.
@@ -200,12 +206,24 @@ def call(
         residual = x
         if self.normalize_first:
             x = self._self_attention_layer_norm(x)
-        x = self._self_attention_layer(
-            query=x,
-            value=x,
-            attention_mask=self_attention_mask,
-            training=training,
-        )
+
+        if return_attention_scores:
+            x, attention_scores = self._self_attention_layer(
+                query=x,
+                value=x,
+                attention_mask=self_attention_mask,
+                return_attention_scores=return_attention_scores,
+                training=training,
+            )
+            return x, attention_scores
+        else:
+            x = self._self_attention_layer(
+                query=x,
+                value=x,
+                attention_mask=self_attention_mask,
+                training=training,
+            )
+
         x = self._self_attention_dropout(x, training=training)
         x = x + residual
         if not self.normalize_first:
@@ -222,6 +240,9 @@ def call(
         if not self.normalize_first:
             x = self._feedforward_layer_norm(x)
 
+        if return_attention_scores:
+            return x, attention_scores
+
         return x
 
     def get_config(self):

keras_hub/src/layers/modeling/transformer_encoder_test.py

@@ -95,3 +95,14 @@ def test_mask_propagation(self):
         inputs._keras_mask = mask
         outputs = encoder(inputs)
         self.assertAllEqual(outputs._keras_mask, mask)
+
+    def test_attention_scores(self):
+        encoder = TransformerEncoder(intermediate_dim=4, num_heads=2)
+        inputs = random.uniform(shape=[1, 4, 6])
+        outputs, attention_scores = encoder(
+            inputs, return_attention_scores=True
+        )
+        self.assertAllEqual(outputs.shape, inputs.shape)
+
+        # attention scores shape (batch_size, num_of_attn_heads, seq_length, seq_length)
+        self.assertAllEqual(attention_scores.shape, [1, 2, 4, 4])