Fix issue #5299. (#5343)

nni/compression/pytorch/speedup/infer_mask.py

@@ -86,11 +86,15 @@ class AutoMaskInference:
             self.output_mask = self.in_masks[0]
         else:
             if isinstance(self.output, torch.Tensor):
+                if self.output.requires_grad:
+                    self.output.retain_grad()   # issue #5299
                 self.output_mask = torch.ones_like(self.output)
             elif isinstance(self.output, list) or isinstance(self.output, tuple):
                 self.output_mask = []
                 for o_tensor in self.output:
                     if isinstance(o_tensor, torch.Tensor):
+                        if o_tensor.requires_grad:
+                            o_tensor.retain_grad()  # issue #5299
                         self.output_mask.append(torch.ones_like(o_tensor))
                     else:
                         # if one of the outputs is not tensor, set the corresponding