Support attn2==None for xformers (#1759)

src/diffusers/models/attention.py

@@ -473,6 +473,7 @@ class BasicTransformerBlock(nn.Module):
             except Exception as e:
                 raise e
             self.attn1._use_memory_efficient_attention_xformers = use_memory_efficient_attention_xformers
+            if self.attn2 is not None:
                 self.attn2._use_memory_efficient_attention_xformers = use_memory_efficient_attention_xformers
 
     def forward(self, hidden_states, encoder_hidden_states=None, timestep=None, attention_mask=None):