Merge with the branch of xuchen (NOT update the float16, this needs code review) and fix the bugs in Gather function.
1. Support Reciprocal fucntion.
2. Fix the safe delete bugs in XDevice.
3. Support new API to convert the data type of tensor.
4. Support to show the memory usage of buffer memory.
5. Fix minor errors.

1. Try to fix the bugs in destroy the stream of XDevice (Uncheck).
2. Fix the bug of memory leak in ReduceSumAll function.
2. Adjust the directory structure.
3. Fix the minor errors.

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.
2. Fix the minor errors in Normalize and ScaleAndShift functions.

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. Call the Scale or Shift functions when the parameter shift is 0.0F or scale is 1.0F.
2. Add the unit test for ScaleAndShift for the cases which parameter shift is 0.0F or scale is 1.0F.

Use the _CheckData without tolerance in some unit test to speedup the test process.

1. Use the _CheckData with tolerance in unit test to avoid the bug in "0.0F != -0.0F" case.
2, Clean the codes.

1. Fix the bug in MultiplyMe function which cannot handle the scalar tensor and broadcast case.
2. Add broadcast multiply unit test.

1. CPU Support for Gather function.
2. Fix the bugs in GPU Gather.
2. Minor error fixed.

1. Supporting efficient propagate and gradient accumulation for backward functions.
2. Update the setData functions.
3. Clean the codes.

Minor error fixed.

1. Support to copy parts of source tensor to parts of target tensor.
2. New unit test for CopyIndex function.

1. Support scalar tensor for Multiply and Div operation;
2. Fix backward bugs in Multiply and Div function;
3. Minor bugs fixed.

New unit test for Sub function.

Minor bugs fixed.

1. Support scalar tensor for Sum operation;
2. Bounds check;
3. GetSumDimIndex function bug fixed;
4. New unit test for Sum function;
2. Minor bugs fixed.

1. Make link for ReduceSumAll function.
2. Fix the memory leak issue.

Use tensor as the bottom implementation and support plain scalar.
Note: DO NOT support the backward for ReduceSumAll function.

1. Initialize and new scalar tensor.
2. Get and set scalar tensor.
3. Add ReduceSum test on scalar tensor.
4. Little bugs fixed.

1. Move the SetAscendingOrder functions from XTensor.*  to core/utilities/SetAscendingOder.*
2. Rename XElement.* files to XCall.*

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.*