Add capability for cmake to fetch libtorch itself if not found

cmake/gauxc-onedft.cmake

@@ -24,7 +24,9 @@ endif()
 
 # store and restore CMAKE_CUDA_ARCHITECTURES if Torch clobbers it
 set(_PREV_CUDA_ARCHS "${CMAKE_CUDA_ARCHITECTURES}")
-find_package(Torch REQUIRED)
+
+include(skala-torch)
+
 if(CMAKE_CUDA_ARCHITECTURES STREQUAL "OFF")
   set(CMAKE_CUDA_ARCHITECTURES "${_PREV_CUDA_ARCHS}" CACHE STRING "Restore CUDA archs after Torch override" FORCE)
   message(WARNING "Torch set CMAKE_CUDA_ARCHITECTURES to OFF. Restored previous value: ${CMAKE_CUDA_ARCHITECTURES}")

cmake/skala-torch.cmake

@@ -0,0 +1,76 @@
+# Find or download LibTorch
+find_package(Torch QUIET)
+
+if(NOT Torch_FOUND)
+    message(STATUS "Torch not found. Downloading libtorch...")
+
+    # Set libtorch version and download URL
+    set(LIBTORCH_VERSION "2.9.1")
+    set(USE_CUDA_LIBTORCH FALSE) #default is not to use the cuda version but cpu version
+    if(GAUXC_HAS_CUDA)
+	find_package(CUDAToolkit)
+        set(SUPPORTED_CUDA_VERSION_NO_DOTS "126" "128" "130")
+        set(CUDA_VERSION_NO_DOT ${CUDAToolkit_VERSION_MAJOR}${CUDAToolkit_VERSION_MINOR})
+        if(CUDA_VERSION_NO_DOT IN_LIST SUPPORTED_CUDA_VERSION_NO_DOTS)
+	    find_package(CUDA REQUIRED) # Needed for Caffe cmake, since it use the old CUDA inclusion
+	    set(USE_CUDA_LIBTORCH TRUE)
+	    message(STATUS "CUDAToolkit_INCLUDE_DIRS: ${CUDAToolkit_INCLUDE_DIRS}")
+	    set(CUDA_INCLUDE_DIRS "${CUDAToolkit_INCLUDE_DIRS}") # and pass over the correct headers to Caffe cmake
+        else()
+	    message(WARNING "CUDA toolkit version is ${CUDAToolkit_VERSION}, for which there is no libtorch to download.")
+	    message(WARNING "Falling back to cpu version of libtorch.")
+        endif()
+    endif()
+
+
+    # Determine the appropriate libtorch variant based on platform and CUDA availability
+    if(UNIX AND NOT APPLE)
+        # Linux
+        if(USE_CUDA_LIBTORCH)
+	    set(LIBTORCH_URL "https://download.pytorch.org/libtorch/cu${CUDA_VERSION_NO_DOT}/libtorch-shared-with-deps-${LIBTORCH_VERSION}%2Bcu${CUDA_VERSION_NO_DOT}.zip")
+        else()
+            set(LIBTORCH_URL "https://download.pytorch.org/libtorch/cpu/libtorch-shared-with-deps-${LIBTORCH_VERSION}%2Bcpu.zip")
+        endif()
+    elseif(APPLE)
+        # macOS (CPU only)
+        set(LIBTORCH_URL "https://download.pytorch.org/libtorch/cpu/libtorch-macos-arm64-${LIBTORCH_VERSION}.zip")
+    elseif(WIN32)
+        # Windows
+        if(USE_CUDA_LIBTORCH)
+	    set(LIBTORCH_URL "https://download.pytorch.org/libtorch/cu${CUDA_VERSION_NO_DOT}/libtorch-win-shared-with-deps-${LIBTORCH_VERSION}%2Bcu${CUDA_VERSION_NO_DOT}.zip")
+        else()
+            set(LIBTORCH_URL "https://download.pytorch.org/libtorch/cpu/libtorch-win-shared-with-deps-${LIBTORCH_VERSION}%2Bcpu.zip")
+        endif()
+    endif()
+
+    set(LIBTORCH_DOWNLOAD_DIR "${CMAKE_BINARY_DIR}/libtorch")
+    set(LIBTORCH_ZIP "${CMAKE_BINARY_DIR}/libtorch.zip")
+
+    # Download libtorch
+    if(NOT EXISTS ${LIBTORCH_DOWNLOAD_DIR})
+        message(STATUS "Downloading libtorch from ${LIBTORCH_URL}")
+        file(DOWNLOAD ${LIBTORCH_URL} ${LIBTORCH_ZIP}
+            SHOW_PROGRESS
+            STATUS DOWNLOAD_STATUS)
+
+        list(GET DOWNLOAD_STATUS 0 STATUS_CODE)
+        if(NOT STATUS_CODE EQUAL 0)
+            message(FATAL_ERROR "Failed to download libtorch: ${DOWNLOAD_STATUS}")
+        endif()
+
+        # Extract libtorch
+        message(STATUS "Extracting libtorch...")
+        file(ARCHIVE_EXTRACT INPUT ${LIBTORCH_ZIP} DESTINATION ${CMAKE_BINARY_DIR})
+
+        # Clean up zip file
+        file(REMOVE ${LIBTORCH_ZIP})
+    endif()
+
+    # Set CMAKE_PREFIX_PATH to find the downloaded libtorch
+    set(CMAKE_PREFIX_PATH "${LIBTORCH_DOWNLOAD_DIR};${CMAKE_PREFIX_PATH}")
+
+    # Find Torch package again
+    find_package(Torch REQUIRED)
+endif()
+
+message(STATUS "Using Torch version: ${Torch_VERSION}")