Check empty before clip grads (#219)

Signed-off-by: lizz <lizz@sensetime.com>

mmcv/runner/hooks/optimizer.py

@@ -11,8 +11,10 @@ class OptimizerHook(Hook):
         self.grad_clip = grad_clip
 
     def clip_grads(self, params):
-        clip_grad.clip_grad_norm_(
-            filter(lambda p: p.requires_grad, params), **self.grad_clip)
+        params = list(
+            filter(lambda p: p.requires_grad and p.grad is not None, params))
+        if len(params) > 0:
+            clip_grad.clip_grad_norm_(params, **self.grad_clip)
 
     def after_train_iter(self, runner):
         runner.optimizer.zero_grad()