executorch: install torch at build time, disable PEP 517 isolation

CMake's find_package_torch_headers/get_torch_base_path resolves torch by
running `import torch` in the same interpreter running the build backend.
torch is a runtime dependency (setup.py), not declared in
[build-system].requires, so pip's isolated build venv never has it and
CMake configure fails with AttributeError: 'NoneType' object has no
attribute 'submodule_search_locations'.

Preinstall build-system.requires plus torch in CIBW_BEFORE_BUILD and
disable isolation via CIBW_BUILD_FRONTEND, mirroring the same pattern
already used in build-torchaudio.yml and build-torchcodec.yml.

Author: luhenry

.github/workflows/build-executorch.yml

@@ -165,6 +165,18 @@ jobs:
           # build dependency; its own CMake bootstrap needs OpenSSL's dev
           # headers to find libcrypto.
           CIBW_BEFORE_ALL_LINUX: dnf -y install openssl-devel
+          # get_torch_base_path (tools/cmake/Utils.cmake) runs `import torch` in
+          # the same interpreter as the build backend to add it to
+          # CMAKE_PREFIX_PATH, but torch is only a runtime dependency
+          # (setup.py), not a build-system.requires entry, so PEP 517 isolation
+          # hides it from that interpreter. Preinstall build-system.requires
+          # plus torch and turn isolation off (mirrors build-torchaudio.yml /
+          # build-torchcodec.yml).
+          CIBW_BUILD_FRONTEND: "pip; args: --no-build-isolation"
+          CIBW_BEFORE_BUILD: >-
+            pip install "cmake>=3.24,<4.0.0" "packaging>=24.2" pyyaml
+            "setuptools>=77.0.3" wheel zstd certifi &&
+            pip install --only-binary=:all: "torch>=2.13.0a0"
           CIBW_ENVIRONMENT: >-
             CMAKE_BUILD_PARALLEL_LEVEL=8
             PIP_EXTRA_INDEX_URL=https://pypi.riseproject.dev/simple/