[mlgo] Don't provide default model URLs

[mtrofin]

Summary

Pointed out in Issue #56432: the current reference models may not be
quite friendly to open source projects. Their purpose is only
illustrative - the expectation is that projects would train their own.
To avoid unintentionally pulling such a model, made the URL cmake
setting require explicit user setting.

[yundiqian]

llvm/cmake/modules/TensorFlowCompile.cmake

97–98

also provide the information about where to find the downloading urls in the error message?

[mtrofin]

also pointing out there are model releases at the given repo

llvm/cmake/modules/TensorFlowCompile.cmake

if (EXISTS "${override_header}" AND EXISTS "${override_object}")
     set(GENERATED_OBJS ${GENERATED_OBJS} ${obj_file})
     set(GENERATED_HEADERS ${GENERATED_HEADERS} ${hdr_file})
   elseif("${model}" STREQUAL "none")
     message(STATUS "Will skip enabling mlgo for ${fname}")
     return()
   else()
     if ("${model}" STREQUAL "download")
       # Crash if the user wants to download a model but a URL is set to "TO_BE_UPDATED"
-      if ("${default_url}" STREQUAL "TO_BE_UPDATED")
-          message(FATAL_ERROR "Default URL was set to 'download' but there is no"
-          " model url currently specified in cmake - likely, the model interface"
-          " recently changed, and so there is not a released model available.")
+      if ("${default_url}" STREQUAL "<UNSPECIFIED>")
+          message(FATAL_ERROR "Model path was set to 'download' but there is no"
+          " model url currently specified in cmake. You can generate a model"
+          " using, for example, the tools at http://github.com/google/ml-compiler-opt."
+          " Some reference models are also periodically released there.")
       endif()
 
       set(model ${default_url})
     endif()
 
     if ("${model}" STREQUAL "autogenerate")
       set(model ${default_path}-autogenerated)
       generate_mock_model(${test_model_generator} ${model})

llvm/lib/Analysis/CMakeLists.txt

 if (DEFINED LLVM_HAVE_TF_AOT OR DEFINED LLVM_HAVE_TF_API)
   include(TensorFlowCompile)
   set(LLVM_INLINER_MODEL_PATH_DEFAULT "models/inliner-Oz")
 
-  # This url points to the most recent most which is known to be compatible with
-  # LLVM. When better models are published, this url should be updated to aid
-  # discoverability.
-  set(LLVM_INLINER_MODEL_CURRENT_URL "https://github.com/google/ml-compiler-opt/releases/download/inlining-Oz-v1.1/inlining-Oz-99f0063-v1.1.tar.gz")
+  set(LLVM_INLINER_MODEL_CURRENT_URL "<UNSPECIFIED>")
 
   if (DEFINED LLVM_HAVE_TF_AOT)
     tf_find_and_compile(
       ${LLVM_INLINER_MODEL_PATH}
       ${LLVM_INLINER_MODEL_CURRENT_URL}
       ${LLVM_INLINER_MODEL_PATH_DEFAULT}
       "models/gen-inline-oz-test-model.py"
       serve

llvm/lib/CodeGen/CMakeLists.txt

 if (DEFINED LLVM_HAVE_TF_AOT OR DEFINED LLVM_HAVE_TF_API)
   include(TensorFlowCompile)
   set(LLVM_RAEVICT_MODEL_PATH_DEFAULT "models/regalloc-eviction")
 
-  # This url points to the most recent most which is known to be compatible with
-  # LLVM. When better models are published, this url should be updated to aid
-  # discoverability.
-  set(LLVM_RAEVICT_MODEL_CURRENT_URL "https://github.com/google/ml-compiler-opt/releases/download/regalloc-evict-v1.0/regalloc-evict-e67430c-v1.0.tar.gz")
+  set(LLVM_RAEVICT_MODEL_CURRENT_URL "<UNSPECIFIED>")
 
   if (DEFINED LLVM_HAVE_TF_AOT)
     tf_find_and_compile(
       ${LLVM_RAEVICT_MODEL_PATH}
       ${LLVM_RAEVICT_MODEL_CURRENT_URL}
       ${LLVM_RAEVICT_MODEL_PATH_DEFAULT}
       "../Analysis/models/gen-regalloc-eviction-test-model.py"
       serve