Bypass find_package(torch) for find_package_torch_headers to help out cross compilation

[swolchok]

To be honest, I don't understand how the Android demo compiles currently. Indeed, it doesn't work on my Mac without this change, but with this change it does.

Should fix #8556

[swolchok]

Stack from ghstack (oldest at bottom):

• build_variables.bzl: split PLATFORM_SRCS from EXECUTORCH_CORE_SRCS #8398
• build_variables.bzl: split PROGRAM_NO_PRIM_OPS_SRCS from EXECUTORCH_CORE_SRCS #8397
• create and validate build_variables.bzl #8326
• determine fht_avx.c/fht_neon.c based on CMake target processor #8346
• Add extension/llm/custom_ops/spinquant/test to CMake #8345
• -> Bypass find_package(torch) for find_package_torch_headers to help out cross compilation #8629

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 71ef8af with merge base f3fc096 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm / linux-job (gh)
  backends/arm/test/ops/test_bmm.py::TestBMM::test_bmm_single_input_tosa_MI_2

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llava-runner-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[swolchok]

error message I was getting for posterity:

CMake Error at .etvenv/lib/python3.12/site-packages/cmake/data/share/cmake-3.31/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find Torch (missing: TORCH_LIBRARY)
Call Stack (most recent call first):
  .etvenv/lib/python3.12/site-packages/cmake/data/share/cmake-3.31/Modules/FindPackageHandleStandardArgs.cmake:603 (_FPHSA_FAILURE_MESSAGE)
  .etvenv/lib/python3.12/site-packages/torch/share/cmake/Torch/TorchConfig.cmake:175 (find_package_handle_standard_args)
  build/Utils.cmake:386 (find_package)
  build/Utils.cmake:344 (find_package_torch)
  CMakeLists.txt:628 (find_package_torch_headers)