support flash-attn at torch backend (#2257)

* support flash-attn at torch backend

* fix

* fix

* fix

* fix conflit

* fix conflit

* fix conflit

* fix conflit

* fix conflit

* fix conflit

* format

Author: pass-lin

keras_hub/src/models/gemma/gemma_attention.py

@@ -152,7 +152,7 @@ def _compute_attention(
                 attention_mask = ops.expand_dims(attention_mask, axis=1)
                 attention_mask = ops.cast(attention_mask, dtype="bool")
             # Only pass soft cap if needed as not all keras versions support.
-            if self.logit_soft_cap:
+            if self.logit_soft_cap is not None:
                 kwargs = {"attn_logits_soft_cap": self.logit_soft_cap}
             else:
                 kwargs = {}

keras_hub/src/models/qwen_moe/qwen_moe_attention.py

@@ -67,6 +67,7 @@ def __init__(
         self.rope_scaling_factor = rope_scaling_factor
         self.use_sliding_window_attention = use_sliding_window_attention
         self.sliding_window_size = sliding_window_size
+        self.logit_soft_cap = None
 
     def build(self, inputs_shape):
         # Einsum variables:

keras_hub/src/utils/keras_utils.py

@@ -71,6 +71,23 @@ def fused_attention_op_available():
             )
             return False
         return True
+    elif (
+        hasattr(keras.config, "is_flash_attention_enabled")
+        and keras.config.backend() == "torch"
+    ):
+        try:
+            from torch.backends.cuda import SDPAParams as SDPAParams
+            from torch.backends.cuda import (
+                can_use_flash_attention as can_use_flash_attention,
+            )
+        except ImportError:
+            logging.warning(
+                "Flash attention is not supported in your current PyTorch "
+                "version. Please update it by following the official guide: "
+                "https://pytorch.org/get-started/locally/"
+            )
+            return False
+        return True
     else:
         return False