For the detail about the API/interface for vendor compilers, it is suggested to take a look at the tutorial.
For AutoTVM, we do not support auto-tuning for BYOC now. LIke @mbaret has mentioned, AutoTVM is targeting to TVM functions. One sentence for AutoTVM is: finding the best config from a tuning space defined in a given TVM schedule function (e.g., TOPI schedule). In other words, AutoTVM cannot figure out a tuning space if the schedule implementation is not in TVM schedule primitives.
In long term, we might plan to propose a representation for vendors to specify a tuning space so that we can leverage AutoTVM to tune the performance for external codegen as well, but we currently do not prioritize this task due to the lack of bandwidth and driving applications.