[Split Build] Fix libtorch_python RPATH (#129088)

In the split build we end up with an incorrect RPATH for `libtorch_python.so`. This PR fixes said RPATH.

What the rpath should look like:
```
sahanp@devgpu086 ~/pytorch ((636de71c…))> objdump -p ~/main_so_files/libtorch_python.so | grep "RPATH"                        (pytorch-3.10)
  RPATH                /lib/intel64:/lib/intel64_win:/lib/win-x64:/home/sahanp/pytorch/build/lib:/home/sahanp/.conda/envs/pytorch-3.10/lib:
```

Before

```
sahanp@devgpu086 ~/pytorch ((636de71c…))> objdump -p ~/split_so_files/libtorch_python.so | grep "RPATH"                       (pytorch-3.10)
  RPATH                /home/sahanp/pytorch/torch/lib:/home/sahanp/pytorch/build/lib:
```

After
```
sahanp@devgpu086 ~/pytorch ((636de71c…))> objdump -p build/lib/libtorch_python.so | grep "RPATH"                              (pytorch-3.10)
  RPATH                /lib/intel64:/lib/intel64_win:/lib/win-x64:/home/sahanp/pytorch/build/lib:/home/sahanp/pytorch/torch/lib:/home/sahanp/.conda/envs/pytorch-3.10/lib:
```

Testing that this works is in the above PR. Similarly, after running ciflow/binaries the output of objdump -p should not change https://www.diffchecker.com/14PRmCNz/ (checked manywheel py 3.10 cuda 12.1)
Pull Request resolved: #129088
Approved by: https://github.com/malfet

.github/workflows/pull.yml

@@ -515,3 +515,29 @@ jobs:
       build-environment: linux-focal-cuda12.1-py3.10-gcc9-experimental-split-build
       docker-image: ${{ needs.linux-focal-cuda12_1-py3_10-gcc9-experimental-split-build.outputs.docker-image }}
       test-matrix: ${{ needs.linux-focal-cuda12_1-py3_10-gcc9-experimental-split-build.outputs.test-matrix }}
+
+  linux-focal-py3_12-clang10-experimental-split-build:
+    name: linux-focal-py3.12-clang10-experimental-split-build
+    uses: ./.github/workflows/_linux-build-label.yml
+    with:
+      use_split_build: True
+      build-environment: linux-focal-py3.12-clang10
+      docker-image-name: pytorch-linux-focal-py3.12-clang10
+      test-matrix: |
+        { include: [
+          { config: "default", shard: 1, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "default", shard: 2, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "default", shard: 3, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "dynamo", shard: 1, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "dynamo", shard: 2, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "dynamo", shard: 3, num_shards: 3, runner: "linux.2xlarge" },
+        ]}
+  linux-focal-py3_12-clang10-experimental-split-build-test:
+    name: linux-focal-py3.12-clang10-experimental-split-build
+    uses: ./.github/workflows/_linux-test.yml
+    needs: linux-focal-py3_12-clang10-experimental-split-build
+    with:
+      build-environment: linux-focal-py3.12-clang10-experimental-split-build
+      docker-image: ${{ needs.linux-focal-py3_12-clang10-experimental-split-build.outputs.docker-image }}
+      test-matrix: ${{ needs.linux-focal-py3_12-clang10-experimental-split-build.outputs.test-matrix }}
+      timeout-minutes: 600

torch/CMakeLists.txt

@@ -298,6 +298,16 @@ endif()
 
 
 add_library(torch_python SHARED ${TORCH_PYTHON_SRCS})
+
+if(CAFFE2_USE_MKL AND BUILD_LIBTORCHLESS)
+
+  # Use the RPATH of the linked libraries
+  set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+  # we need to explicitly link caffe2::mkl in order to have the
+  # correct RPATH in torch_python for the split build
+  target_link_libraries(torch_python PRIVATE caffe2::mkl)
+endif()
+
 add_dependencies(torch_python onnx_proto)
 # Avoid numpy for the DEPLOY build
 if(USE_NUMPY)