Fix multiple find_package(Torch) calls

[huydhn]

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

[pytorch-bot]

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[huydhn]

This issue looks strange though and I think it might better fix it upstream in https://github.com/pytorch/pytorch/blob/main/cmake/public/mkl.cmake. @malfet Have you seen this issue before?

[swolchok]

thank you!!!