`variable_scope` use `auxiliary_name_scope` to control whether to create new name scope (#14390)

* TST: add test case

* ENH: variable_scope supports auxiliary_name_scope

* DOC: add document

* ENH: support to reuse name scope given by user

* TST: add test for name scope

* DOC: revise docment for name scope

* CLN: revise TypeError for None

* TST: add test for reentering variable_scope

* BUG: root scope reuse name scope

* TST: add test case for root scope

* TST: remove name scope tests

* ENH: remove name scope support

* DOC: remove name scope support

* TST: split and add more test cases

* BUG: root name scope is empty string

* CLN: remove comment