Update LTC CMake hack documentation (#1155)

* Update CMakeLists.txt

* Update CMakeLists.txt

* Update CMakeLists.txt

* Update CMakeLists.txt

* Update buildAndTest.yml

* Update setup.py

* Address review comments

.github/workflows/buildAndTest.yml

@@ -87,6 +87,7 @@ jobs:
           externals/llvm-project/llvm
         cmake --build llvm-build
 
+        # TODO: Reenable LTC once OOT build is successful (https://github.com/llvm/torch-mlir/issues/1154)
         cmake -GNinja -Bbuild \
           -DCMAKE_LINKER=lld \
           -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \

python/CMakeLists.txt

@@ -87,7 +87,8 @@ declare_mlir_python_extension(TorchMLIRPythonExtensions.Main
 if(TORCH_MLIR_ENABLE_LTC)
   add_subdirectory(torch_mlir/csrc/base_lazy_backend)
 endif()
-# Reference backend has a separate check for TORCH_MLIR_ENABLE_LTC.
+# Reference backend has a separate check for TORCH_MLIR_ENABLE_LTC, since it
+# generates a dummy Python library when disabled.
 add_subdirectory(torch_mlir/csrc/reference_lazy_backend)
 
 ################################################################################

python/torch_mlir/dialects/torch/importer/jit_ir/csrc/CMakeLists.txt

@@ -10,7 +10,8 @@ include_directories(BEFORE
   )
 link_directories("${TORCH_INSTALL_PREFIX}/lib")
 
-# Hack! Currently out-of-tree build fails when this is set to SHARED, so we have this toggle
+# TODO: Currently, out-of-tree build fails when LIBRARY_TYPE is set to SHARED, so we have this toggle.
+#       See https://github.com/llvm/torch-mlir/issues/1154 for more details.
 if(TORCH_MLIR_ENABLE_LTC)
   set(LIBRARY_TYPE "SHARED")
 else()

setup.py

@@ -76,6 +76,7 @@ class CMakeBuild(build_py):
                 f"-DMLIR_ENABLE_BINDINGS_PYTHON=ON",
                 f"-DLLVM_ENABLE_PROJECTS=mlir",
                 f"-DTORCH_MLIR_ENABLE_MHLO=ON",
+                # TODO: Reenable LTC once JIT importer linkage issue is fixed (https://github.com/llvm/torch-mlir/issues/1154)
                 f"-DTORCH_MLIR_ENABLE_LTC=OFF",
                 f"-DLLVM_EXTERNAL_PROJECTS=torch-mlir;torch-mlir-dialects",
                 f"-DLLVM_EXTERNAL_TORCH_MLIR_SOURCE_DIR={src_dir}",