Target arch in ti.aot.Module

[ailzhang]

Although we currently support specifying target arch when creating a ti.aot.Module it's no-op and can be misleading.. Properly supporting that requires a refactor to separrate compilation and runtime, so practically we should remove this option in the short term and ask users to properly specify the target arch in the ti.init() for now.

Separating compilation and runtime is something that kinda work in progress throughout the AOT support so we should be able to add that feature back in a few releases.