Fix compile error when ninja not available and custon kernel activated

src/transformers/models/rt_detr/modeling_rt_detr.py

@@ -899,7 +899,8 @@ def forward(
         else:
             raise ValueError(f"Last dim of reference_points must be 2 or 4, but got {reference_points.shape[-1]}")
 
-        if self.disable_custom_kernels:
+        # Ignore copy
+        if self.disable_custom_kernels or MultiScaleDeformableAttention is None:
             # PyTorch implementation
             output = multi_scale_deformable_attention(
                 value, spatial_shapes_list, sampling_locations, attention_weights