* Chage `IndicateDataType` to `GetKernelType`. Make it easier to
  understand.
* Change `OpKernelKey` to `OpKernelType`
* Make operator developers can customize which kernel the operator will
  use in runtime.

* polish p ops

* fix precision_recall

* fix linear_chain_crf_op

* small fix

* proj init.

* add unittest and implementation.