[1/n] Migrate permute_cols to libtorch stable ABI (#31509)

Signed-off-by: Mikayla Gawarecki <mikaylagawarecki@gmail.com>

CMakeLists.txt

@@ -340,7 +340,6 @@ if(VLLM_GPU_LANG STREQUAL "CUDA")
 
   list(APPEND VLLM_EXT_SRC
     "csrc/quantization/awq/gemm_kernels.cu"
-    "csrc/permute_cols.cu"
     "csrc/quantization/w8a8/cutlass/scaled_mm_entry.cu"
     "csrc/quantization/fp4/nvfp4_quant_entry.cu"
     "csrc/quantization/fp4/nvfp4_scaled_mm_entry.cu"
@@ -986,6 +985,48 @@ define_extension_target(
 # Setting this variable sidesteps the issue by calling the driver directly.
 target_compile_definitions(_C PRIVATE CUTLASS_ENABLE_DIRECT_CUDA_DRIVER_CALL=1)
 
+# add OR VLLM_GPU_LANG STREQUAL "HIP" here once
+# https://github.com/vllm-project/vllm/issues/35163 is resolved
+if(VLLM_GPU_LANG STREQUAL "CUDA")
+  #
+  # _C_stable_libtorch extension (ops registered via STABLE_TORCH_LIBRARY)
+  #
+  set(VLLM_STABLE_EXT_SRC
+    "csrc/libtorch_stable/torch_bindings.cpp")
+
+  if(VLLM_GPU_LANG STREQUAL "CUDA")
+    list(APPEND VLLM_STABLE_EXT_SRC "csrc/libtorch_stable/permute_cols.cu")
+  endif()
+
+  if(VLLM_GPU_LANG STREQUAL "CUDA")
+    set_gencode_flags_for_srcs(
+      SRCS "${VLLM_STABLE_EXT_SRC}"
+      CUDA_ARCHS "${CUDA_ARCHS}")
+  endif()
+
+  message(STATUS "Enabling C_stable extension.")
+  define_extension_target(
+    _C_stable_libtorch
+    DESTINATION vllm
+    LANGUAGE ${VLLM_GPU_LANG}
+    SOURCES ${VLLM_STABLE_EXT_SRC}
+    COMPILE_FLAGS ${VLLM_GPU_FLAGS}
+    ARCHITECTURES ${VLLM_GPU_ARCHES}
+    USE_SABI 3
+    WITH_SOABI)
+
+  # Set TORCH_TARGET_VERSION for stable ABI compatibility.
+  # This ensures we only use C-shim APIs available in PyTorch 2.10.
+  # _C_stable_libtorch is abi compatible with PyTorch >= TORCH_TARGET_VERSION
+  # which is currently set to 2.10.
+  target_compile_definitions(_C_stable_libtorch PRIVATE
+    TORCH_TARGET_VERSION=0x020A000000000000ULL)
+
+  # Needed to use cuda APIs from C-shim
+  target_compile_definitions(_C_stable_libtorch PRIVATE
+    USE_CUDA)
+endif()
+
 #
 # _moe_C extension
 #