fix hrnet import Uniform (#3859)

ppdet/modeling/backbones/hrnet.py

@@ -15,10 +15,10 @@
 import paddle
 import paddle.nn as nn
 import paddle.nn.functional as F
-from paddle.nn import AdaptiveAvgPool2D, Linear, Uniform
+from paddle.nn import AdaptiveAvgPool2D, Linear
 from paddle.regularizer import L2Decay
 from paddle import ParamAttr
-from paddle.nn.initializer import Normal
+from paddle.nn.initializer import Normal, Uniform
 from numbers import Integral
 import math