[CMake] Fix TORCH_CUDA_ARCH_LIST warning (#104680)

The warning complains that `TORCH_CUDA_ARCH_LIST` is set on the environment
instead of being defined as a build variable, which is fixed by the change to
`tools/setup_helpers/cmake.py`.

However, I still see the warning even with this fix because
```cmake
if((NOT EXISTS ${TORCH_CUDA_ARCH_LIST}) ...
```
is actually checking whether a file exists called "7.5" (or whatever arch is
being requested). Instead we want to check if the variable is defined.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104680
Approved by: https://github.com/albanD

cmake/public/utils.cmake

@@ -381,7 +381,7 @@ endmacro()
 #
 macro(torch_cuda_get_nvcc_gencode_flag store_var)
   # setting nvcc arch flags
-  if((NOT EXISTS ${TORCH_CUDA_ARCH_LIST}) AND (DEFINED ENV{TORCH_CUDA_ARCH_LIST}))
+  if((NOT DEFINED TORCH_CUDA_ARCH_LIST) AND (DEFINED ENV{TORCH_CUDA_ARCH_LIST}))
     message(WARNING
         "In the future we will require one to explicitly pass "
         "TORCH_CUDA_ARCH_LIST to cmake instead of implicitly setting it as an "
@@ -389,7 +389,7 @@ macro(torch_cuda_get_nvcc_gencode_flag store_var)
         "pytorch.")
     set(TORCH_CUDA_ARCH_LIST $ENV{TORCH_CUDA_ARCH_LIST})
   endif()
-  if(EXISTS ${CUDA_ARCH_NAME})
+  if(DEFINED CUDA_ARCH_NAME)
     message(WARNING
         "CUDA_ARCH_NAME is no longer used. Use TORCH_CUDA_ARCH_LIST instead. "
         "Right now, CUDA_ARCH_NAME is ${CUDA_ARCH_NAME} and "

tools/setup_helpers/cmake.py

@@ -233,6 +233,7 @@ class CMake:
                     "OPENSSL_ROOT_DIR",
                     "STATIC_DISPATCH_BACKEND",
                     "SELECTED_OP_LIST",
+                    "TORCH_CUDA_ARCH_LIST",
                     "TRACING_BASED",
                 )
             }