init of op diag (#5298)

* init of op diag

* init of op diag

* modify  op diag

* merge testcase

* delete no use tensor diag

* amend diag api docs

* resolve conficts

* resolve confict

* resolve confict

* resolve confict

* auto format by CI

* add tensor.diag

* auto format by CI

* amend API description

* add non_square test

* amend diag API description

* amend diag API description

* compact test code

* compact code

* auto format by CI

* amend API description

* amend API description

* motify functional API

* motify test_diag

* amend diag functional API

* amend diag docstring

* auto format by CI

* amend code standards

* auto format by CI

Co-authored-by: oneflow-ci-bot <ci-bot@oneflow.org>
Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>