* upgrade gather_tree to core.ops (#30697)

* upgrade gather_tree to core.ops

* update gather_tree unittests

* update gather_tree doc (#30693)

* update gather_tree doc, test=document_fix

* update sample code, test=document_fix

* remove tensor type, test=document_fix

* fix error info for rnn related api

* passed local test, test=develop

* double check the code

* double check the code, test=develop

* update 'shape' check in RNNCell, test=develop

* add long dtype to RNNCell

* fix long type in python3
Co-authored-by: NXingWu01 <wuxing@iie.ac.cn>

There are PythonAPI tests in Op's unittest which don't need to inherit OpTest class.