Allow disabling torch 2_0 attention (huggingface#3273)

* Allow disabling torch 2_0 attention

* make style

* Update src/diffusers/models/attention.py

Author: patrickvonplaten

src/diffusers/models/attention.py

@@ -71,6 +71,7 @@ def __init__(
         self.proj_attn = nn.Linear(channels, channels, bias=True)
 
         self._use_memory_efficient_attention_xformers = False
+        self._use_2_0_attn = True
         self._attention_op = None
 
     def reshape_heads_to_batch_dim(self, tensor, merge_head_and_batch=True):
@@ -142,9 +143,8 @@ def forward(self, hidden_states):
 
         scale = 1 / math.sqrt(self.channels / self.num_heads)
 
-        use_torch_2_0_attn = (
-            hasattr(F, "scaled_dot_product_attention") and not self._use_memory_efficient_attention_xformers
-        )
+        _use_2_0_attn = self._use_2_0_attn and not self._use_memory_efficient_attention_xformers
+        use_torch_2_0_attn = hasattr(F, "scaled_dot_product_attention") and _use_2_0_attn
 
         query_proj = self.reshape_heads_to_batch_dim(query_proj, merge_head_and_batch=not use_torch_2_0_attn)
         key_proj = self.reshape_heads_to_batch_dim(key_proj, merge_head_and_batch=not use_torch_2_0_attn)