set use_ema to True in ppyolo series' tutorials

dygraph/tutorials/train/object_detection/ppyolo.py

@@ -55,5 +55,6 @@ model.train(
     warmup_start_lr=0.0,
     save_interval_epochs=5,
     lr_decay_epochs=[243, 324],
+    use_ema=True,
     save_dir='output/ppyolo_r50vd_dcn',
     use_vdl=True)

dygraph/tutorials/train/object_detection/ppyolotiny.py

@@ -54,6 +54,7 @@ model.train(
     warmup_steps=1000,
     warmup_start_lr=0.0,
     lr_decay_epochs=[430, 540, 610],
+    use_ema=True,
     save_interval_epochs=5,
     save_dir='output/ppyolotiny',
     use_vdl=True)

dygraph/tutorials/train/object_detection/ppyolov2.py

@@ -58,5 +58,6 @@ model.train(
     warmup_steps=1000,
     warmup_start_lr=0.0,
     lr_decay_epochs=[243],
+    use_ema=True,
     save_interval_epochs=5,
     save_dir='output/ppyolov2_r50vd_dcn')