* Add diag grad kernel

* fix unittest case

* add float16, remove const &

* delete diag_grad in op_utils.h

* replace prefix pt by pd

* replace added kernel

* revert util change

* pd kernel to phi

* resolve conflict

* resolve conflict

* rename pten dir to phi

* rename namespace to phi

* rename infrt pten dir to phi

* resolve conflict

* rename pten to phi in cmake

* revert all infrt change

* change needed files

* fix infrt failed

* fix inference failed

* move operators/math/math_function_* to pten/kernels/func
* namespace from `paddle::operators::math` to `pten::funcs`

* Fix typos in english docs for diag and diagflat.

Fix diag OP bug on Windows Python3.8 ，remove the std::min

add paddle.tensor.linalg.diag API, diag_v2 OP and CUDA kernel.