Keep the caller's numpy when setting up the Core ML backend

backends/apple/coreml/scripts/install_requirements.sh clones coremltools
and installs its reqs/build.pip before building the mlmodel library.
That file pins numpy==2.0.0 for Python 3.9 to 3.12, so running the script
silently downgrades numpy in the caller's environment.

pandas 3.1.0 (released October 22, 2026) requires numpy>=2.0.2, so after
the downgrade `import pandas` fails, and anything that imports the
ExecuTorch devtools (for example examples/apple/coreml/scripts/export.py)
breaks with "Please upgrade numpy to >= 2.0.2". Most Core ML jobs in CI
hit this.

The coremltools 9.0 CMake build only needs numpy (it asks numpy for its
include path) and setuptools (it builds the bundled kmeans1d helper with
setup.py). Install just those, unpinned, so an existing numpy is left
alone. This also fixes a plain Python 3.12 venv, which has no setuptools:
there the old script stopped at the CMake configure step.

The comment next to COREMLTOOLS_VERSION now points at setup.py, where the
coremltools pin lives, and asks to re-check this package list on a bump.

Test plan:
- Python 3.12 venv with numpy 2.4.6, pandas 3.1.0 and setuptools, script
  before and after this change. Before: mlmodel builds, numpy drops to
  2.0.0, `import pandas` fails. After: mlmodel builds, numpy stays 2.4.6,
  `import pandas` works.
- Brand new Python 3.12 venv with only pip, CMake 3.31: the script
  installs numpy and setuptools, builds mlmodel and copies the protobuf
  sources.
- Configuring coremltools with only numpy fails at kmeans1d, and with
  only setuptools fails at the numpy include path, so both are needed.

Author: shoumikhin

backends/apple/coreml/scripts/install_requirements.sh

@@ -13,7 +13,7 @@ SCRIPT_DIR_PATH="$(
 )"
 
 # TODO(jathu): remove the need to fetch coremltools to build deps for coreml_executor_runner.
-# Keep this version in sync with: pyproject.toml
+# Keep this version in sync with setup.py, and re-check the pip install below when bumping it.
 COREMLTOOLS_VERSION="9.0"
 
 # Safe colors (no TERM noise in CI)
@@ -75,12 +75,14 @@ if [[ "$(uname)" == "Darwin" ]]; then
 fi
 
 echo "${green}ExecuTorch: Installing coremltools dependencies.${reset}"
+# Not coremltools' reqs/build.pip, whose old numpy pin would downgrade the caller's numpy.
+# Its CMake build only needs numpy (for the include path) and setuptools (to build kmeans1d).
 SDKROOT="$HOST_SDKROOT" \
 CC="$HOST_CC" \
 CXX="$HOST_CXX" \
 CFLAGS="$HOST_CFLAGS" \
 CXXFLAGS="$HOST_CXXFLAGS" \
-python -m pip install -r "$COREMLTOOLS_DIR_PATH/reqs/build.pip"
+python -m pip install numpy setuptools
 STATUS=$?
 if [ $STATUS -ne 0 ]; then
     echo "${red}ExecuTorch: Failed to install coremltools dependencies.${reset}"