Fix multiple find_package(Torch) calls (#8407)

While debugging the build issue on #8322 w.r.t mkl, I undercover a complex interaction between #8322, #8248 (to install mkl), and https://github.com/pytorch/pytorch/blob/main/cmake/public/mkl.cmake from PyTorch.  The error is as follows:

```
CMake Error at /opt/conda/envs/py_3.10/lib/cmake/mkl/MKLConfig.cmake:744 (add_library): <-- This file comes from conda mkl
  add_library cannot create imported target "MKL::MKL" because another target
  with the same name already exists.
Call Stack (most recent call first):
  /opt/conda/envs/py_3.10/lib/python3.10/site-packages/torch/share/cmake/Caffe2/public/mkl.cmake:1 (find_package) <-- this is from PyTorch
  /opt/conda/envs/py_3.10/lib/python3.10/site-packages/torch/share/cmake/Caffe2/Caffe2Config.cmake:106 (include)
  /opt/conda/envs/py_3.10/lib/python3.10/site-packages/torch/share/cmake/Torch/TorchConfig.cmake:68 (find_package)
  CMakeLists.txt:753 (find_package)
```

The conclusion is that, with mkl installed, there should be just one `find_package(Torch)` call because the mkl target is defined globally.  The `torch` target, on the other hand, is only defined locally.

So, this change adds `if(NOT TARGET torch)` check to only call `find_package(Torch)` if needed.

### Testing

The change on top of #8322 looks like this f705b01

https://github.com/pytorch/executorch/actions/runs/13278590926?pr=8399

CMakeLists.txt

@@ -750,7 +750,9 @@ if(EXECUTORCH_BUILD_PYBIND)
   endif()
 
   # find pytorch lib, to allow pybind to take at::Tensor as input/output
-  find_package(Torch CONFIG REQUIRED)
+  if(NOT TARGET torch)
+    find_package(Torch CONFIG REQUIRED)
+  endif()
   find_library(
     TORCH_PYTHON_LIBRARY torch_python PATHS "${TORCH_INSTALL_PREFIX}/lib"
   )

build/Codegen.cmake

@@ -146,7 +146,9 @@ function(gen_custom_ops_aot_lib)
     ${_out_dir}/CustomOpsNativeFunctions.h "${GEN_KERNEL_SOURCES}"
   )
   # Find `Torch`.
-  find_package(Torch REQUIRED)
+  if(NOT TARGET torch)
+    find_package(Torch REQUIRED)
+  endif()
   # This lib uses ATen lib, so we explicitly enable rtti and exceptions.
   target_compile_options(${GEN_LIB_NAME} PRIVATE -frtti -fexceptions)
   target_compile_definitions(${GEN_LIB_NAME} PRIVATE USE_ATEN_LIB=1)

extension/llm/custom_ops/CMakeLists.txt

@@ -69,7 +69,9 @@ install(TARGETS custom_ops DESTINATION lib)
 
 if(EXECUTORCH_BUILD_KERNELS_CUSTOM_AOT)
   # Add a AOT library
-  find_package(Torch CONFIG REQUIRED)
+  if(NOT TARGET torch)
+    find_package(Torch CONFIG REQUIRED)
+  endif()
   add_library(
     custom_ops_aot_lib SHARED
     ${_custom_ops__srcs}