Install examples and domain libraries as last (optional) step (#11653)

Install requirements is installing so many things that's not necessary
for core executorch runtime. For example, it's not necessary to install
packages that is used in examples.

Ideally, developers should just run to install core ET. 

```
install_requirement.sh
pip install . -e
```

Author: mergennachin

install_executorch.py

@@ -16,6 +16,7 @@
 from contextlib import contextmanager
 
 from install_requirements import (
+    install_optional_example_requirements,
     install_requirements,
     python_is_compatible,
     TORCH_NIGHTLY_URL,
@@ -157,6 +158,13 @@ def _parse_args() -> argparse.Namespace:
         "picked up without rebuilding the wheel. Extension libraries will be "
         "installed inside the source tree.",
     )
+    parser.add_argument(
+        "--minimal",
+        "-m",
+        action="store_true",
+        help="Only installs necessary dependencies for core executorch and skips "
+        " packages necessary for running example scripts.",
+    )
     return parser.parse_args()
 
 
@@ -182,9 +190,13 @@ def main(args):
     # This option is used in CI to make sure that PyTorch build from the pinned commit
     # is used instead of nightly. CI jobs wouldn't be able to catch regression from the
     # latest PT commit otherwise
-    install_requirements(use_pytorch_nightly=not args.use_pt_pinned_commit)
-    os.execvp(
-        sys.executable,
+    use_pytorch_nightly = not args.use_pt_pinned_commit
+
+    # Step 1: Install core dependencies first
+    install_requirements(use_pytorch_nightly)
+
+    # Step 2: Install core package
+    cmd = (
         [
             sys.executable,
             "-m",
@@ -198,8 +210,13 @@ def main(args):
             "-v",
             "--extra-index-url",
             TORCH_NIGHTLY_URL,
-        ],
+        ]
     )
+    subprocess.run(cmd, check=True)
+
+    # Step 3: Extra (optional) packages that is only useful for running examples.
+    if not args.minimal:
+        install_optional_example_requirements(use_pytorch_nightly)
 
 
 if __name__ == "__main__":

install_requirements.py

@@ -76,27 +76,15 @@ def python_is_compatible():
 
 def install_requirements(use_pytorch_nightly):
     # pip packages needed by exir.
-    EXIR_REQUIREMENTS = [
+    TORCH_PACKAGE = [
         # Setting use_pytorch_nightly to false to test the pinned PyTorch commit. Note
         # that we don't need to set any version number there because they have already
         # been installed on CI before this step, so pip won't reinstall them
         f"torch==2.8.0.{NIGHTLY_VERSION}" if use_pytorch_nightly else "torch",
-        (
-            f"torchvision==0.23.0.{NIGHTLY_VERSION}"
-            if use_pytorch_nightly
-            else "torchvision"
-        ),  # For testing.
     ]
 
-    EXAMPLES_REQUIREMENTS = [
-        f"torchaudio==2.8.0.{NIGHTLY_VERSION}" if use_pytorch_nightly else "torchaudio",
-    ]
-
-    # Assemble the list of requirements to actually install.
-    # TODO: Add options for reducing the number of requirements.
-    REQUIREMENTS_TO_INSTALL = EXIR_REQUIREMENTS + EXAMPLES_REQUIREMENTS
-
-    # Install the requirements. `--extra-index-url` tells pip to look for package
+    # Install the requirements for core ExecuTorch package.
+    # `--extra-index-url` tells pip to look for package
     # versions on the provided URL if they aren't available on the default URL.
     subprocess.run(
         [
@@ -105,10 +93,8 @@ def install_requirements(use_pytorch_nightly):
             "pip",
             "install",
             "-r",
-            "requirements-examples.txt",
-            "-r",
             "requirements-dev.txt",
-            *REQUIREMENTS_TO_INSTALL,
+            *TORCH_PACKAGE,
             "--extra-index-url",
             TORCH_NIGHTLY_URL,
         ],
@@ -139,15 +125,61 @@ def install_requirements(use_pytorch_nightly):
     )
 
 
+def install_optional_example_requirements(use_pytorch_nightly):
+    print("Installing packages in requirements-examples.txt")
+    subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "pip",
+            "install",
+            "-r",
+            "requirements-examples.txt",
+        ],
+        check=True,
+    )
+
+    print("Installing torch domain libraries")
+    DOMAIN_LIBRARIES = [
+        (
+            f"torchvision==0.23.0.{NIGHTLY_VERSION}"
+            if use_pytorch_nightly
+            else "torchvision"
+        ),
+        f"torchaudio==2.8.0.{NIGHTLY_VERSION}" if use_pytorch_nightly else "torchaudio",
+    ]
+    # Then install domain libraries
+    subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "pip",
+            "install",
+            *DOMAIN_LIBRARIES,
+            "--extra-index-url",
+            TORCH_NIGHTLY_URL,
+        ],
+        check=True,
+    )
+
+
 def main(args):
     parser = argparse.ArgumentParser()
     parser.add_argument(
         "--use-pt-pinned-commit",
         action="store_true",
         help="build from the pinned PyTorch commit instead of nightly",
     )
+    parser.add_argument(
+        "--example",
+        action="store_true",
+        help="Also installs required packages for running example scripts.",
+    )
     args = parser.parse_args(args)
-    install_requirements(use_pytorch_nightly=not bool(args.use_pt_pinned_commit))
+    use_pytorch_nightly=not bool(args.use_pt_pinned_commit)
+    install_requirements(use_pytorch_nightly)
+    if args.example:
+        install_optional_example_requirements(use_pytorch_nightly)
 
 
 if __name__ == "__main__":

requirements-dev.txt

@@ -1,7 +1,11 @@
+# Pip packages needed to build from source. Mainly for development of ExecuTorch.
+
 cmake>=3.19, <4.0.0  # For building binary targets in the wheel.
 pip>=23  # For building the pip package.
 pyyaml  # Imported by the kernel codegen tools.
 setuptools>=63  # For building the pip package contents.
 tomli  # Imported by extract_sources.py when using python < 3.11.
 wheel  # For building the pip package archive.
 zstd  # Imported by resolve_buck.py.
+lintrunner==0.12.7
+lintrunner-adapters==0.12.4

requirements-lintrunner.txt

@@ -1,7 +1,3 @@
-# Lintrunner itself
-lintrunner==0.12.7
-lintrunner-adapters==0.12.4
-
 # Flake 8 and its dependencies
 flake8==6.1.0
 flake8-breakpoint==1.1.0