Is it considered cheating if you post assignment questions on a site but no one answered it
At my university, the answer is definitely yes. The rules here clearly state that even the attempt to cheat is misconduct (e.g. bringing a device that is capable of cheating to an exam is considered cheating even if it not used/touched).

(However, if asking a question about your assignment in some forum is cheating, depends on the rules for your course/institution. In my courses, this would not be forbidden for homework assignments, for example.)