* feat: added polygamma init code

* feat: added polygamma unittest code

* test: added more test cases

* refactor: added forward impl

* refactor: added backward impl

* test: updated cases

* refactor: updated test cases

* refactor: added more case and fixed some bugs

* test: updated ref func

* refactor: updated code style

* refactor: move the code

* refactor: updated test

* refactor: updated test

* docs: updated en doc
Co-authored-by: Nzachary sun <70642955+sunzhongkai588@users.noreply.github.com>

* docs: updated math eq

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Co-authored-by: Nzachary sun <70642955+sunzhongkai588@users.noreply.github.com>