Make MPS op imports side-effect safe during package discovery

[saisharan0103]

Fixes #4566.

pkgutil.walk_packages(torchao.__path__) can import torchao.experimental.ops.mps while discovering packages even when the caller is not trying to use MPS functionality. On XPU/Windows wheels, the Python MPS package is present, but the MPS custom ops and libtorchao_ops_mps_aten.dylib are not available. The unconditional _load_torchao_mps_lib() call in torchao.experimental.ops.mps.__init__ raises RuntimeError, and pkgutil.walk_packages only suppresses ImportError, so package enumeration aborts.

This makes the MPS package import side-effect safe by gating the eager native library load behind a small availability helper in torchao/experimental/ops/mps/utils.py. Non-macOS/non-MPS environments can import the package without registering native ops or raising from the dylib lookup, while the existing explicit loader remains available for actual MPS use.

Adds a regression test in test/test_lean_import.py that verifies pkgutil.walk_packages(torchao.__path__) completes without an unhandled RuntimeError.

ruff check test/test_lean_import.py torchao/experimental/ops/mps/__init__.py torchao/experimental/ops/mps/utils.py reports no new findings on the changed files.
Ran pytest -x locally with no new failures.

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