Added an empty __init__.py file to the MLIR Python bindings

[superbobry]

Summary

While not strictly required after PEP-420, it is better to have one, since not
all tooling supports implicit namespace packages.

[stellaraccident]

Could we revert this and discuss? This source tree was designed explicitly to be namespace packaged, and iirc, there are cases where that is important: with an init file at the root, the entire tree must come from one directory. Further, since no changes were made to the build files, I don't even see how this is doing anything.

I'm not opposed to deciding differently or learning something new, but the description provides no material to do so. If re-applying, please include notes about which tools or workflows are having problems and reference any source for why "it is better to have one".

While this may look cosmetic, adding init files to namespace packaged is a functional change.

mlir/python/mlir/__init__.py

+# Licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception