Update install_requirements to install --pybind xnnpack by default.

Summary:

Regular ./install_requirements.sh will install pybind xnnpack by default. It is necessary for Llama for instance.

It is still backwards compatible with './install_requirements.sh --pybind xnnpack'

One can also turn off explicitly by calling './install_requirement.sh --pybind off'

Test Plan:

Test valid options:

./install_requirements.sh
./install_requirements.sh --pybind xnnpack
./install_requirements.sh --pybind coreml
./install_requirements.sh --pybind coreml xnnpack
./install_requirements.sh --pybind off

Invalid options:

./install_requirements.sh xnnpack
./install_requirements.sh --pybind coreml off
./install_requirements.sh --pybind coreml xnnpack off
./install_requirements.sh off

install_requirements.py

@@ -66,21 +66,34 @@ def python_is_compatible():
     sys.exit(1)
 
 # Parse options.
-EXECUTORCH_BUILD_PYBIND = "OFF"
+
+EXECUTORCH_BUILD_PYBIND = ""
 CMAKE_ARGS = os.getenv("CMAKE_ARGS", "")
 CMAKE_BUILD_ARGS = os.getenv("CMAKE_BUILD_ARGS", "")
 USE_PYTORCH_NIGHTLY = True
 
-for arg in sys.argv[1:]:
+args = sys.argv[1:]
+for arg in args:
     if arg == "--pybind":
-        EXECUTORCH_BUILD_PYBIND = "ON"
+        pass
     elif arg in ["coreml", "mps", "xnnpack"]:
-        if EXECUTORCH_BUILD_PYBIND == "ON":
+        if "--pybind" in args:
             arg_upper = arg.upper()
+            EXECUTORCH_BUILD_PYBIND = "ON"
             CMAKE_ARGS += f" -DEXECUTORCH_BUILD_{arg_upper}=ON"
         else:
             print(f"Error: {arg} must follow --pybind")
             sys.exit(1)
+    elif arg == "off":
+        if "--pybind" in args:
+            if EXECUTORCH_BUILD_PYBIND == "ON":
+                print("Cannot turnoff pybind option as it is already set.")
+                sys.exit(1)
+            EXECUTORCH_BUILD_PYBIND = "OFF"
+        else:
+            print(f"Error: {arg} must follow --pybind")
+            sys.exit(1)
+
     elif arg == "--clean":
         print("Cleaning build artifacts...")
         print("Cleaning pip-out/...")
@@ -100,6 +113,13 @@ def python_is_compatible():
         print(f"Error: Unknown option {arg}")
         sys.exit(1)
 
+# If --pybind is not set explicitly for backends (e.g., --pybind xnnpack)
+# or is not turned off explicitly (--pybind off)
+# then install XNNPACK by default.
+if EXECUTORCH_BUILD_PYBIND == "":
+    EXECUTORCH_BUILD_PYBIND = "ON"
+    CMAKE_ARGS += " -DEXECUTORCH_BUILD_XNNPACK=ON"
+
 # Use ClangCL on Windows.
 # ClangCL is an alias to Clang that configures it to work in an MSVC-compatible
 # mode. Using it on Windows to avoid compiler compatibility issues for MSVC.