Add an interface to set the number of threads for math function, and set the default value to 1 for inference.

* Fix a bug related to dispensable inputs and refine the inference unittest.

* Fix the use of dispensable inputs in reshape_op.

* Polish the enforce statements.

* Fix an English writing typo.

- Added hash function inside of MKLDNN softmax op to be used as handle for primitives stroing in a
context

- Style fixes to softmax mkldnn op

- Fixes after review

- Coding style

- Fix to style

- style fixes

- style fix

- style fixes

- Fix to cody style check

- Rephrasing a comment

* Add FP16 option in save_combine_op

* Fix casting issue

Do not use ctor

* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.

Rename MatDim to MatDescriptor

* adding operator to blas deps list

* use device_context instead to solve cycle deps

* "optimizer op support float64"

* "fix ci"

* "fix ftrl op"