Support fp16 data type for more operations and fix the minor errors.  (Don't use this! It's an incomplete version)

1. Fix the mistakes in manual. By the way, I have to say there are so many mistakes in the manual. I'm shocked it has been checked a lot of times, but why they are still be there. No one care about that? Really???
2. Clean the codes.

1. Move the New/Initial/Del Tensor functions from XTensor.* to XElement.*
2. Move IsSameShaped functions from XTensor.* to core/shape/IsSameShaped.*
3. Move CheckData functions from XTensor.* to core/utilities/CheckData.*

1. Merge with Huchi branch (replace all requireLink with enableGrad that allows gradient computation for a tensor);
2. Update the global memory size (This may make the memory size a little bit larger than the old version).

1. add macro to implement unary function 2. add sub and div function 3. merge code with the latest branch of xiaotong-working