Enable Flash Attention for SD3 MMDiT (#2014)

* Enable flash attention for SD3 MMDiT

* Remove tf condition

keras_hub/src/models/stable_diffusion_3/mmdit.py

@@ -595,9 +595,28 @@ def build(self, inputs_shape, context_shape, timestep_embedding_shape):
         self.context_block.build(context_shape, timestep_embedding_shape)
 
     def _compute_attention(self, query, key, value):
+        batch_size = ops.shape(query)[0]
+
+        # Use the fast path when `ops.dot_product_attention` and flash attention
+        # are available.
+        if hasattr(ops, "dot_product_attention") and hasattr(
+            keras.config, "is_flash_attention_enabled"
+        ):
+            # `ops.dot_product_attention` is slower than the vanilla
+            # implementation in the tensorflow backend.
+            encoded = ops.dot_product_attention(
+                query,
+                key,
+                value,
+                scale=self._inverse_sqrt_key_dim,
+                flash_attention=keras.config.is_flash_attention_enabled(),
+            )
+            return ops.reshape(
+                encoded, (batch_size, -1, self.num_heads * self.head_dim)
+            )
+
         # Ref: jax.nn.dot_product_attention
         # https://github.com/jax-ml/jax/blob/db89c245ac66911c98f265a05956fdfa4bc79d83/jax/_src/nn/functions.py#L846
-        batch_size = ops.shape(query)[0]
         logits = ops.einsum("BTNH,BSNH->BNTS", query, key)
         logits = ops.multiply(logits, self._inverse_sqrt_key_dim)
         probs = self.softmax(logits)