fix attention mask pad check (#3531)

Author: williamberman

src/diffusers/models/attention_processor.py

@@ -381,12 +381,7 @@ def prepare_attention_mask(self, attention_mask, target_length, batch_size=None,
             return attention_mask
 
         current_length: int = attention_mask.shape[-1]
-        if current_length > target_length:
-            # we *could* trim the mask with:
-            #   attention_mask = attention_mask[:,:target_length]
-            # but this is weird enough that it's more likely to be a mistake than a shortcut
-            raise ValueError(f"mask's length ({current_length}) exceeds the sequence length ({target_length}).")
-        elif current_length < target_length:
+        if current_length != target_length:
             if attention_mask.device.type == "mps":
                 # HACK: MPS: Does not support padding by greater than dimension of input tensor.
                 # Instead, we can manually construct the padding tensor.