Add __version__ attribute for programmatic version checking (#5046)

Summary:
Pull Request resolved: #5046

Adds `__version__` to `ax/__init__.py` by importing from `ax.version`, which is generated by `setuptools_scm` at build time. Uses a try/except fallback to `"Unknown"` for environments where the version file hasn't been generated yet, matching the same pattern used in BoTorch. This enables programmatic version checking via `ax.__version__`.

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AI generated Summary & Test Plan from DEV111709377

Reviewed By: saitcakmak

Differential Revision: D96685150

fbshipit-source-id: 76f7bd8885ab67e9630b012e33b20b2110d69f78

Author: Eytan Bakshy

ax/__init__.py

@@ -10,6 +10,11 @@
 from ax.api.configs import ChoiceParameterConfig, RangeParameterConfig, StorageConfig
 from ax.api.types import TOutcome, TParameterization
 
+try:
+    from ax.version import version as __version__  # @manual
+except Exception:  # pragma: no cover
+    __version__: str = "Unknown"
+
 __all__ = [
     "Client",
     "ChoiceParameterConfig",