change ppyolo tutorial training configuration

dygraph/tutorials/train/object_detection/ppyolo.py

@@ -46,12 +46,13 @@ model = pdx.models.PPYOLO(num_classes=num_classes, backbone='ResNet50_vd_dcn')
 # API说明：https://github.com/PaddlePaddle/PaddleX/blob/release/2.0-rc/paddlex/cv/models/detector.py#L154
 # 各参数介绍与调整说明：https://paddlex.readthedocs.io/zh_CN/develop/appendix/parameters.html
 model.train(
-    num_epochs=405,
+    num_epochs=450,
     train_dataset=train_dataset,
     train_batch_size=8,
     eval_dataset=eval_dataset,
+    pretrain_weights='COCO',
     learning_rate=0.005 / 12,
-    warmup_steps=1000,
+    warmup_steps=500,
     warmup_start_lr=0.0,
     save_interval_epochs=5,
     lr_decay_epochs=[243, 324],