Add warning when no param update (#4896)

* style(Optim): add warning when no param update

* style(Optim): add TODO
Co-authored-by: Noneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

oneflow/python/nn/optimizer/optimizer.py

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 """
 
+import warnings
 from typing import Dict, Callable, Union, Any, Iterator
 from types import GeneratorType
 
@@ -71,10 +72,20 @@ class Optimizer(object):
         raise NotImplementedError()
 
     def zero_grad(self, set_to_none: bool = False):
+        all_grad_is_none = True
         for param_group in self._param_groups:
             for param in param_group.parameters:
-                if set_to_none:
-                    param.grad = None
-                else:
-                    param.grad.fill_(0)
-                    # param.grad.zeros_()
+                if param.grad is not None:
+                    all_grad_is_none = False
+                    if set_to_none:
+                        param.grad = None
+                    else:
+                        param.grad.fill_(0)
+                        # param.grad.zeros_()
+        if all_grad_is_none:
+            # TODO: delete this after implementing Tensor.data
+            warnings.warn(
+                "\nParameters in optimizer do not have gradient.\n"
+                "Please check `loss.backward()` is called or not,\n"
+                "or try to declare optimizer after calling `module.to()`"
+            )