fix nan in optimizer_on_cpu

examples/run_squad.py

@@ -687,11 +687,12 @@ def set_optimizer_params_grad(named_params_optimizer, named_params_model, test_n
         if name_opti != name_model:
             logger.error("name_opti != name_model: {} {}".format(name_opti, name_model))
             raise ValueError
-        if test_nan and torch.isnan(param_model.grad).sum() > 0:
-            is_nan = True
-        if param_opti.grad is None:
-            param_opti.grad = torch.nn.Parameter(param_opti.data.new().resize_(*param_opti.data.size()))
-        param_opti.grad.data.copy_(param_model.grad.data)
+        if param_model.grad is not None:
+            if test_nan and torch.isnan(param_model.grad).sum() > 0:
+                is_nan = True
+            if param_opti.grad is None:
+                param_opti.grad = torch.nn.Parameter(param_opti.data.new().resize_(*param_opti.data.size()))
+            param_opti.grad.data.copy_(param_model.grad.data)
     return is_nan
 
 def main():