Abnormal diff of gradients in unitest for CrossChannelNormLayer

[pkuyym]

Gradients calculation or backward processing of CrossChannelNormLayer maybe incorrect.

27: I0523 19:34:54.603669 3745645504 LayerGradUtil.cpp:643] layer_type=norm useGpu=0
27: I0523 19:34:54.604331 3745645504 LayerGradUtil.cpp:683] cost 115.088
27: I0523 19:34:54.604535 3745645504 LayerGradUtil.cpp:43] cross-channel-norm para_0 step=1e-06 cost1=115.099 cost2=115.077 true_delta=0.0222626 analytic_delta=0.0111297 diff=1.00029 ***
27: I0523 19:34:54.604568 3745645504 LayerGradUtil.cpp:50] The previous diff might be caused by not accumulating parameter gradients in backward()
27: I0523 19:34:54.604655 3745645504 LayerGradUtil.cpp:43] cross-channel-norm layer_0 step=1.37416e-05 cost1=115.086 cost2=115.09 true_delta=-0.00338745 analytic_delta=0.00115088 diff=-3.94335 ***
27: I0523 19:34:54.604702 3745645504 LayerGradUtil.cpp:643] layer_type=norm useGpu=0
27: I0523 19:34:54.604790 3745645504 LayerGradUtil.cpp:683] cost -107.057
27: I0523 19:34:54.605002 3745645504 LayerGradUtil.cpp:43] cross-channel-norm layer_0 step=-1.30582e-05 cost1=-107.055 cost2=-107.058 true_delta=0.00298309 analytic_delta=-0.00107057 diff=-3.78646 ***