[REFACTOR] Phase out relay python components (apache#17656)

This PR starts the step 0 to phase out relay from the current
development main branch.  This PR focuses on the python
components of relay, autotvm, auto_scheduler. To make the change
manageable, we will also do followup steps on te.Schedule and
c++ components in followup PRs.

To continue support community members who depends on
legacy flows, the [v0.19.0](https://github.com/apache/tvm/tree/v0.19.0)
branch will continue contain these components.


As noted in [discussion on phasing out legacy components](https://discuss.tvm.apache.org/t/phasing-out-legacy-components/17703/30),
this would help us to do two purposes:

- By removing outdated or redundant elements, we can significantly
reduce complexity and improve maintainability.
- Unify our focus: Concentrating our efforts on the new unity flow
will allow for more efficient development and innovation.

It is also a good opportunity for us to revisit and reduce CI time.
The past relay legacy flow contains a lot of end to end tests that
requires hardware resources to run and causing long CI time.
Moving onwards, we can focus more on unit-tests that focuses
on structural equality and runs within seconds, while be mindful
about tests that requires hardware resources (by restricting them
to specific folders and CI nightly in some cases).

---

Co-authored-by: Siyuan Feng <hzfengsy@sjtu.edu.cn>

Author: 2 people

CMakeLists.txt

@@ -829,6 +829,7 @@ if(NOT DEFINED ENV{CONDA_BUILD})
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/python
   )
   add_dependencies(tvm_cython tvm)
+  message("Add Cython build into the default build step")
 endif()
 
 # Installation rules