Add modified huber loss operator

Complete smooth_l1_loss_op.

Fix a few typos in docs

update F&Q.

remove unused XXXLayer.h in Layer.cpp

Refine platform::Transform function and fix prelu_op testing.

Add the support of the cross-compiling for iOS devices

add MKLDNN relu and tanh

Add soft-label support for cross-entropy operator.

Fix compile error in prelu_op.

Add dropout operator.