[modenbert] fix regression (#39750)

* fix regression

* add FA2 test

src/transformers/models/modernbert/modeling_modernbert.py

@@ -19,6 +19,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import copy
 import math
 from contextlib import nullcontext
 from typing import Optional, Union
@@ -459,20 +460,21 @@ class ModernBertAttention(nn.Module):
 
         if layer_id % config.global_attn_every_n_layers != 0:
             self.local_attention = (config.local_attention // 2, config.local_attention // 2)
+            rope_theta = config.local_rope_theta if config.local_rope_theta is not None else config.global_rope_theta
+            max_position_embeddings = config.local_attention
         else:
             self.local_attention = (-1, -1)
-
             max_position_embeddings = config.max_position_embeddings
-        if self.local_attention != (-1, -1):
+            rope_theta = config.global_rope_theta
-            rope_theta = config.global_rope_theta if config.local_rope_theta is None else config.local_rope_theta
-            max_position_embeddings = config.local_attention
 
         if config._attn_implementation == "flash_attention_2":
             self.rotary_emb = ModernBertUnpaddedRotaryEmbedding(
                 dim=self.head_dim, max_seqlen=max_position_embeddings, base=rope_theta
             )
         else:
-            self.rotary_emb = ModernBertRotaryEmbedding(config=config)
+            config_copy = copy.deepcopy(config)
+            config_copy.rope_theta = rope_theta
+            self.rotary_emb = ModernBertRotaryEmbedding(config=config_copy)
 
         self.Wo = nn.Linear(config.hidden_size, config.hidden_size, bias=config.attention_bias)
         self.out_drop = nn.Dropout(config.attention_dropout) if config.attention_dropout > 0.0 else nn.Identity()
@@ -611,7 +613,9 @@ class ModernBertPreTrainedModel(PreTrainedModel):
             if module.bias is not None:
                 module.bias.data.zero_()
 
-    def set_attention_implementation(self, attn_implementation: Union[dict, str]):
+    def _check_and_adjust_attn_implementation(
+        self, attn_implementation: Optional[str], is_init_check: bool = False
+    ) -> str:
         """
         Checks and dispatches to hhe requested attention implementation.
         """
@@ -620,16 +624,17 @@ class ModernBertPreTrainedModel(PreTrainedModel):
         # ModernBert's FA2 implementation correctly handles non-fp16/bf16 dtypes, we don't
         # need the FA2 warning for non-fp16/bf16 dtypes so we set fp16 for the FA2 check.
 
-        requested_attn_implementation = self._check_attn_implementation(attn_implementation)
         try:
             attn_implementation = (
                 "flash_attention_2"
-                if requested_attn_implementation is None and self._flash_attn_2_can_dispatch()
+                if attn_implementation is None and self._flash_attn_2_can_dispatch()
                 else attn_implementation
             )
         except (ValueError, ImportError):
             pass
-        return super().set_attention_implementation(attn_implementation=attn_implementation)
+        return super()._check_and_adjust_attn_implementation(
+            attn_implementation=attn_implementation, is_init_check=is_init_check
+        )
 
     def _maybe_set_compile(self):
         if self.config.reference_compile is False:

src/transformers/models/modernbert/modular_modernbert.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import copy
 import math
 from contextlib import nullcontext
 from typing import Literal, Optional, Union
@@ -659,20 +660,21 @@ class ModernBertAttention(nn.Module):
 
         if layer_id % config.global_attn_every_n_layers != 0:
             self.local_attention = (config.local_attention // 2, config.local_attention // 2)
+            rope_theta = config.local_rope_theta if config.local_rope_theta is not None else config.global_rope_theta
+            max_position_embeddings = config.local_attention
         else:
             self.local_attention = (-1, -1)
-
             max_position_embeddings = config.max_position_embeddings
-        if self.local_attention != (-1, -1):
+            rope_theta = config.global_rope_theta
-            rope_theta = config.global_rope_theta if config.local_rope_theta is None else config.local_rope_theta
-            max_position_embeddings = config.local_attention
 
         if config._attn_implementation == "flash_attention_2":
             self.rotary_emb = ModernBertUnpaddedRotaryEmbedding(
                 dim=self.head_dim, max_seqlen=max_position_embeddings, base=rope_theta
             )
         else:
-            self.rotary_emb = ModernBertRotaryEmbedding(config=config)
+            config_copy = copy.deepcopy(config)
+            config_copy.rope_theta = rope_theta
+            self.rotary_emb = ModernBertRotaryEmbedding(config=config_copy)
 
         self.Wo = nn.Linear(config.hidden_size, config.hidden_size, bias=config.attention_bias)
         self.out_drop = nn.Dropout(config.attention_dropout) if config.attention_dropout > 0.0 else nn.Identity()
@@ -811,7 +813,9 @@ class ModernBertPreTrainedModel(PreTrainedModel):
             if module.bias is not None:
                 module.bias.data.zero_()
 
-    def set_attention_implementation(self, attn_implementation: Union[dict, str]):
+    def _check_and_adjust_attn_implementation(
+        self, attn_implementation: Optional[str], is_init_check: bool = False
+    ) -> str:
         """
         Checks and dispatches to hhe requested attention implementation.
         """
@@ -820,16 +824,17 @@ class ModernBertPreTrainedModel(PreTrainedModel):
         # ModernBert's FA2 implementation correctly handles non-fp16/bf16 dtypes, we don't
         # need the FA2 warning for non-fp16/bf16 dtypes so we set fp16 for the FA2 check.
 
-        requested_attn_implementation = self._check_attn_implementation(attn_implementation)
         try:
             attn_implementation = (
                 "flash_attention_2"
-                if requested_attn_implementation is None and self._flash_attn_2_can_dispatch()
+                if attn_implementation is None and self._flash_attn_2_can_dispatch()
                 else attn_implementation
             )
         except (ValueError, ImportError):
             pass
-        return super().set_attention_implementation(attn_implementation=attn_implementation)
+        return super()._check_and_adjust_attn_implementation(
+            attn_implementation=attn_implementation, is_init_check=is_init_check
+        )
 
     def _maybe_set_compile(self):
         if self.config.reference_compile is False:

tests/models/modernbert/test_modeling_modernbert.py

@@ -375,6 +375,16 @@ class ModernBertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCa
                 config_dict = json.load(f)
             self.assertNotIn("reference_compile", config_dict)
 
+    @require_flash_attn
+    @require_torch_gpu
+    @pytest.mark.flash_attn_test
+    def test_flash_attention_dispatches_by_defaul(self):
+        "ModernBert should dispatch to FA2 by default, not SDPA"
+        config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
+        for model_class in self.all_model_classes:
+            model = model_class(config=config)
+            self.assertTrue(model.config._attn_implementation == "flash_attention_2")
+
 
 @require_torch
 class ModernBertModelIntegrationTest(unittest.TestCase):