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.

Rename MatDim to MatDescriptor

* adding operator to blas deps list

* use device_context instead to solve cycle deps

* Fix CPPLint issyes in gru_kernel.h

* Fix CPPLint issyes in gru_kernel.h

* Fix Compile error

* fix __shfl_down_sync_ of cross_entropy

* use reduceSum

* "fix ci"

* Fix cpplint issues in sequence_padding

* Fix typo in cu file

* Fix dependencies of sequence_padding

* Add include

* Fix CPPLint issues in lstm_cpu_kernel.h

* Fix CPPLint issues in math/math_function_test

* Fix CPPLint issues in math/math_function_test

* Fix CPPLint issues in math/concat.cc

* Fix CPPLint issues in math/concat.cc

* Fix CPPLint issues in math/concat.cc

* Fix CPPLint issues in math/gru_cpu_kernel

* Fix CPPLint issues in math/selected_rows_functor_test.cu

* Fix compile error

* Fix compile error

* "re-commit "

* "picked up"

* "fix ci"

* "fix pdb hang up issue in cuda 9"

* Fix CPPLint errors

* Fix CPPLint errors in sequence2batch

* Fix compilation

* Fix LSTM op and GRU op

* Fix LSTMP op

* Fix more cpplint errors in operators/math

* Address Code review feedback

* Fix CPPLint issue in test_engine

* Fix CPPLint errors in operators/math

* Fix compilation

* Fix more CPPLint issues

* Fix more CPPLint issues

* Fix more CPPLint issues

* Fix CPPLint issues in operators/math and operators/reader

stride should be int64_t, not int