* Add digamma_op and unittest

* add digamma_op api

* remove special DigammaCudaKernel and correct some docs

* remove unused headers

* fix api doc error

* run pre-commit

* use HOST or DEVICE instead of HOSTDEVICE in implementation of lgamma op

* add test for fp32

* add lgamma to op_threshold_white_list

* add cuda kernel for lgamma kernel

* modify numeric grad delta

* fix small English issue

* change LaunchElementwiseCudaKernel to LaunchSameDimsElementwiseCudaKernel

* rewrite abs op

* rewrite abs op and remove abs in activation

* remove abs register in old codes

* fix abs_grad type error

* fix abs double_grad output name error

* modify abs_grad, abs_grad_grad functor for windows building

* format code style

* fix the bug of result is nan when the divisor is zero

* add missing abs attr and add abs for float16