Automatically put torch in CMAKE_PREFIX_PATH so users/scripts don't have to

[swolchok]

Removed all explicit setting of CMAKE_PREFIX_PATH to the torch directory in favor of automatically doing it when we find_package(Torch). Hat tip to @huydhn for showing the way to only find torch once.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Automatically put torch in CMAKE_PREFIX_PATH so users/scripts don't have to #8474

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8474

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 38720f4 with merge base a01571f ():

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph

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[swolchok]

this one should wait for fully green CI before landing since I'm messing with CI scripts

[larryliu0820]

Thank you for doing this