use torch.matmul instead of einsum in attnetion. (#445)

* use torch.matmul instead of einsum

* fix softmax

src/diffusers/models/attention.py

@@ -275,11 +275,9 @@ class CrossAttention(nn.Module):
         for i in range(hidden_states.shape[0] // slice_size):
             start_idx = i * slice_size
             end_idx = (i + 1) * slice_size
-            attn_slice = (
-                torch.einsum("b i d, b j d -> b i j", query[start_idx:end_idx], key[start_idx:end_idx]) * self.scale
-            )
+            attn_slice = torch.matmul(query[start_idx:end_idx], key[start_idx:end_idx].transpose(1, 2)) * self.scale
             attn_slice = attn_slice.softmax(dim=-1)
-            attn_slice = torch.einsum("b i j, b j d -> b i d", attn_slice, value[start_idx:end_idx])
+            attn_slice = torch.matmul(attn_slice, value[start_idx:end_idx])
 
             hidden_states[start_idx:end_idx] = attn_slice