Add codegen_util.load_generated_function

Defines the new function load_generated_function which is
meant to be a more user friendly version of `load_generated_package`
(which will be particularly more friendly should we no longer re-export
all sub-modules of generated packages).

Also, explains the arguments of `load_generated_package` in its
doc-string.

As an example of the expected usage of this function, for `co` a
`Codegen` object:
``` python3
generated_paths = co.generate_function()

func = load_generated_function(co.name, generated_paths.function_dir)
```

Tested in `test/symforce_codegen_util_test.py`

symforce/codegen/codegen_util.py

@@ -543,6 +543,13 @@ def _load_generated_package_internal(name: str, path: Path) -> T.Tuple[T.Any, T.
 def load_generated_package(name: str, path: T.Openable) -> T.Any:
     """
     Dynamically load generated package (or module).
+
+    name is the full name of the package or module to load (for example,
+    "pkg.sub_pkg" for a package called "sub_pkg" inside of another package
+    "pkg", or "pkg.sub_pkg.mod" for a module called "mod" inside of pkg.sub_pkg).
+
+    path is the path to the directory (or __init__.py) of the package, or the python
+    file of the module.
     """
     # NOTE(brad): We remove all possibly conflicting modules from the cache. This is
     # to ensure that when name is executed, it loads local modules (if any) rather
@@ -574,6 +581,25 @@ def load_generated_package(name: str, path: T.Openable) -> T.Any:
     return module
 
 
+def load_generated_function(func_name: str, path_to_package: T.Openable) -> T.Any:
+    """
+    Loads the function with name func_name found inside the package located at
+    path_to_package.
+
+    Preconditions:
+        path_to_package is a python package with an `__init__.py` containing a module
+            defined in `func_name.py` which in turn defines an attribute named `func_name`.
+            Note: the precondition will be satisfied if the package was generated by
+            `Codegen.generate_function` from a `Codegen` function with name `func_name`.
+    """
+    pkg_path = Path(path_to_package)
+    if pkg_path.name == "__init__.py":
+        pkg_path = pkg_path.parent
+    pkg_name = pkg_path.name
+    func_module = load_generated_package(f"{pkg_name}.{func_name}", pkg_path / f"{func_name}.py")
+    return getattr(func_module, func_name)
+
+
 def load_generated_lcmtype(
     package: str, type_name: str, lcmtypes_path: T.Union[str, Path]
 ) -> T.Type:

test/symforce_codegen_util_test.py

@@ -10,6 +10,10 @@
 from symforce.codegen import codegen_util
 from symforce.test_util import TestCase
 
+PKG_LOCATIONS = Path(__file__).parent / "test_data" / "codegen_util_test_data"
+RELATIVE_PATH = Path("example_pkg", "__init__.py")
+PACKAGE_NAME = "example_pkg"
+
 
 class SymforceCodegenUtilTest(TestCase):
     """
@@ -22,32 +26,50 @@ def test_load_generated_package(self) -> None:
             codegen_util.load_generated_package
         """
 
-        pkg_locations = Path(__file__).parent / "test_data" / "codegen_util_test_data"
-
-        relative_path = Path("example_pkg", "__init__.py")
-
-        package_name = "example_pkg"
-
         pkg1 = codegen_util.load_generated_package(
-            name=package_name, path=pkg_locations / "example_pkg_1" / relative_path
+            name=PACKAGE_NAME, path=PKG_LOCATIONS / "example_pkg_1" / RELATIVE_PATH
         )
 
         # Testing that the module was loaded correctly
         self.assertEqual(pkg1.package_id, 1)
         self.assertEqual(pkg1.sub_module.sub_module_id, 1)
 
         # Testing that sys.modules was not polluted
-        self.assertFalse(package_name in sys.modules)
+        self.assertFalse(PACKAGE_NAME in sys.modules)
 
         pkg2 = codegen_util.load_generated_package(
-            name=package_name, path=pkg_locations / "example_pkg_2" / relative_path
+            name=PACKAGE_NAME, path=PKG_LOCATIONS / "example_pkg_2" / RELATIVE_PATH
         )
 
         # Testing that the module was loaded correctly when a module with the same name has
         # already been loaded
         self.assertEqual(pkg2.package_id, 2)
         self.assertEqual(pkg2.sub_module.sub_module_id, 2)
 
+    def test_load_generated_function(self) -> None:
+        """
+        Tests:
+            codegen_util.load_generated_function
+        """
+
+        func1 = codegen_util.load_generated_function(
+            func_name="func", path_to_package=PKG_LOCATIONS / "example_pkg_1" / RELATIVE_PATH
+        )
+
+        # Testing that the function was loaded correctly
+        self.assertEqual(func1(), 1)
+
+        # Testing that sys.modules was not polluted
+        self.assertFalse(PACKAGE_NAME in sys.modules)
+
+        func2 = codegen_util.load_generated_function(
+            func_name="func", path_to_package=PKG_LOCATIONS / "example_pkg_2" / RELATIVE_PATH
+        )
+
+        # Testing that the function was loaded correctly when a function of the same name
+        # has already been loaded.
+        self.assertEqual(func2(), 2)
+
 
 if __name__ == "__main__":
     TestCase.main()

test/test_data/codegen_util_test_data/example_pkg_1/example_pkg/func.py

@@ -0,0 +1,8 @@
+# ----------------------------------------------------------------------------
+# SymForce - Copyright 2022, Skydio, Inc.
+# This source code is under the Apache 2.0 license found in the LICENSE file.
+# ----------------------------------------------------------------------------
+
+
+def func() -> int:
+    return 1

test/test_data/codegen_util_test_data/example_pkg_2/example_pkg/func.py

@@ -0,0 +1,8 @@
+# ----------------------------------------------------------------------------
+# SymForce - Copyright 2022, Skydio, Inc.
+# This source code is under the Apache 2.0 license found in the LICENSE file.
+# ----------------------------------------------------------------------------
+
+
+def func() -> int:
+    return 2