[Dygraph] add yolov3 voc configs for dygraph (#2337)

* add yolov3 voc configs for dygraph

* simplify yolov3 darknet voc configs

* support num_classes shared from dataset config

dygraph/configs/yolov3/yolov3_darknet53_270e_voc.yml

+_BASE_: [
+  '../datasets/voc.yml',
+  '../runtime.yml',
+  '_base_/optimizer_270e.yml',
+  '_base_/yolov3_darknet53.yml',
+  '_base_/yolov3_reader.yml',
+]
+
+snapshot_epoch: 5
+weights: output/yolov3_darknet53_270e_voc/model_final

dygraph/ppdet/data/reader.py

@@ -43,9 +43,11 @@ class Compose(object):
         for t in self.transforms:
             for k, v in t.items():
                 op_cls = getattr(transform, k)
-                self.transforms_cls.append(op_cls(**v))
-                if hasattr(op_cls, 'num_classes'):
-                    op_cls.num_classes = num_classes
+                f = op_cls(**v)
+                if hasattr(f, 'num_classes'):
+                    f.num_classes = num_classes
+
+                self.transforms_cls.append(f)
 
     def __call__(self, data):
         for f in self.transforms_cls:
@@ -109,8 +111,6 @@ class BatchCompose(Compose):
 
 
 class BaseDataLoader(object):
-    __share__ = ['num_classes']
-
     def __init__(self,
                  inputs_def=None,
                  sample_transforms=[],
@@ -194,6 +194,8 @@ class BaseDataLoader(object):
 
 @register
 class TrainReader(BaseDataLoader):
+    __shared__ = ['num_classes']
+
     def __init__(self,
                  inputs_def=None,
                  sample_transforms=[],
@@ -211,6 +213,8 @@ class TrainReader(BaseDataLoader):
 
 @register
 class EvalReader(BaseDataLoader):
+    __shared__ = ['num_classes']
+
     def __init__(self,
                  inputs_def=None,
                  sample_transforms=[],
@@ -228,6 +232,8 @@ class EvalReader(BaseDataLoader):
 
 @register
 class TestReader(BaseDataLoader):
+    __shared__ = ['num_classes']
+
     def __init__(self,
                  inputs_def=None,
                  sample_transforms=[],