fixed sync_batchnorm export error and add detect_onnx.py

[DefTruth]

• fixed sync_batchnorm export error and add detect_onnx.py.
• see export onnx error  #32
• all test done

my log:

Checking 0/3...
Checking 1/3...
Checking 2/3...
input:  NodeArg(name='images', type='tensor(float)', shape=[1, 3, 320, 320])
output:  NodeArg(name='output', type='tensor(float)', shape=[1, 6375, 85])
output:  NodeArg(name='910', type='tensor(float)', shape=[1, 3, 40, 40, 85])
output:  NodeArg(name='944', type='tensor(float)', shape=[1, 3, 20, 20, 85])
output:  NodeArg(name='978', type='tensor(float)', shape=[1, 3, 10, 10, 85])
output:  NodeArg(name='1012', type='tensor(float)', shape=[1, 3, 5, 5, 85])
ONNX export success, saved as ./weights/yolor-p6-320-320.onnx

@WongKinYiu