[NNAPI] Create get_next_operand_id (#54700)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54700
This is an internal method just to make it more clear what
len(self.operands) is doing.
Test Plan: Unit test
Reviewed By: axitkhurana
Differential Revision: D27536794
Pulled By: dreiss
fbshipit-source-id: 678cee8a47df6757dd2e6feabf2560fd82d32e26
