support sparse evaluation (#5122)

ppdet/engine/trainer.py

@@ -130,6 +130,7 @@ class Trainer(object):
             self.lr = create('LearningRate')(steps_per_epoch)
             self.optimizer = create('OptimizerBuilder')(self.lr, self.model)
 
+            # Unstructured pruner is only enabled in the train mode.
             if self.cfg.get('unstructured_prune'):
                 self.pruner = create('UnstructuredPruner')(self.model,
                                                            steps_per_epoch)