Add pybind extension to the DLL search path on Windows (#14446)

### Summary
When installing ExecuTorch from a wheel on Windows, the native pybinding
extension DLL fails to load
(#14443). This is because
it's not on the search path. From a quick Google search, it seems like
the recommended way to handle this is to call `os.add_dll_directory`
prior to loading the extension.

I tested this locally by installing from wheel in a clean env and then
patching the installed copy in site-packages. With this change, I was
able to import executorch.extension.pybindings.portable_lib. I also ran
the XNNPACK add op tests as a sanity check, which use pybindings.

Author: GregoryComer

extension/pybindings/portable_lib.py

@@ -13,6 +13,9 @@
     This API is experimental and subject to change without notice.
 """
 
+import logging
+import os
+import sys
 import warnings as _warnings
 
 import executorch.exir._warnings as _exir_warnings
@@ -28,6 +31,21 @@
 # dependencies.
 import torch as _torch
 
+logger = logging.getLogger(__name__)
+
+# Update the DLL search path on Windows. This is the recommended way to handle native
+# extensions.
+if sys.platform == "win32":
+    try:
+        # The extension DLL should be in the same directory as this file.
+        pybindings_dir = os.path.dirname(os.path.abspath(__file__))
+        os.add_dll_directory(pybindings_dir)
+    except Exception as e:
+        logger.error(
+            "Failed to add the pybinding extension DLL to the search path. The extension may not work.",
+            e,
+        )
+
 # Let users import everything from the C++ _portable_lib extension as if this
 # python file defined them. Although we could import these dynamically, it
 # wouldn't preserve the static type annotations.