fix downsample bug in MidResTemporalBlock1D (#10250)

Author: holmosaint

src/diffusers/models/unets/unet_1d_blocks.py

@@ -217,7 +217,7 @@ def forward(self, hidden_states: torch.Tensor, temb: torch.Tensor) -> torch.Tens
         if self.upsample:
             hidden_states = self.upsample(hidden_states)
         if self.downsample:
-            self.downsample = self.downsample(hidden_states)
+            hidden_states = self.downsample(hidden_states)
 
         return hidden_states