Re-add xformers enable to UNet2DCondition (#1627)

* finish

* fix

* Update tests/models/test_models_unet_2d.py

* style

Co-authored-by: Anton Lozhkov <anton@huggingface.co>

src/diffusers/modeling_utils.py

@@ -188,6 +188,39 @@ class ModelMixin(torch.nn.Module):
         if self._supports_gradient_checkpointing:
             self.apply(partial(self._set_gradient_checkpointing, value=False))
 
+    def set_use_memory_efficient_attention_xformers(self, valid: bool) -> None:
+        # Recursively walk through all the children.
+        # Any children which exposes the set_use_memory_efficient_attention_xformers method
+        # gets the message
+        def fn_recursive_set_mem_eff(module: torch.nn.Module):
+            if hasattr(module, "set_use_memory_efficient_attention_xformers"):
+                module.set_use_memory_efficient_attention_xformers(valid)
+
+            for child in module.children():
+                fn_recursive_set_mem_eff(child)
+
+        for module in self.children():
+            if isinstance(module, torch.nn.Module):
+                fn_recursive_set_mem_eff(module)
+
+    def enable_xformers_memory_efficient_attention(self):
+        r"""
+        Enable memory efficient attention as implemented in xformers.
+
+        When this option is enabled, you should observe lower GPU memory usage and a potential speed up at inference
+        time. Speed up at training time is not guaranteed.
+
+        Warning: When Memory Efficient Attention and Sliced attention are both enabled, the Memory Efficient Attention
+        is used.
+        """
+        self.set_use_memory_efficient_attention_xformers(True)
+
+    def disable_xformers_memory_efficient_attention(self):
+        r"""
+        Disable memory efficient attention as implemented in xformers.
+        """
+        self.set_use_memory_efficient_attention_xformers(False)
+
     def save_pretrained(
         self,
         save_directory: Union[str, os.PathLike],

tests/models/test_models_unet_2d.py

@@ -30,6 +30,7 @@ from diffusers.utils import (
     torch_all_close,
     torch_device,
 )
+from diffusers.utils.import_utils import is_xformers_available
 from parameterized import parameterized
 
 from ..test_modeling_common import ModelTesterMixin
@@ -255,6 +256,20 @@ class UNet2DConditionModelTests(ModelTesterMixin, unittest.TestCase):
         inputs_dict = self.dummy_input
         return init_dict, inputs_dict
 
+    @unittest.skipIf(
+        torch_device != "cuda" or not is_xformers_available(),
+        reason="XFormers attention is only available with CUDA and `xformers` installed",
+    )
+    def test_xformers_enable_works(self):
+        init_dict, inputs_dict = self.prepare_init_args_and_inputs_for_common()
+        model = self.model_class(**init_dict)
+
+        model.enable_xformers_memory_efficient_attention()
+
+        assert (
+            model.mid_block.attentions[0].transformer_blocks[0].attn1._use_memory_efficient_attention_xformers
+        ), "xformers is not enabled"
+
     @unittest.skipIf(torch_device == "mps", "Gradient checkpointing skipped on MPS")
     def test_gradient_checkpointing(self):
         # enable deterministic behavior for gradient checkpointing