Keep optional modules lazy during IPython autoreload - #3084

What does this PR do?

Fixes #2995.

With %autoreload 2, IPython inspects __file__ on modules in sys.modules. For SpeechBrain's lazy redirects, this forces optional imports: import speechbrain produces a post-execution callback error when Flair is not installed.

Keep __file__ absent until the lazy target has actually loaded. Ordinary attribute access still loads the target, after which its real filename is available. This applies to absolute/relative lazy imports and deprecated redirects without adding an IPython-specific caller check or a dependency.

This intentionally changes an explicit __file__ lookup on an unloaded proxy to raise AttributeError; the class documentation now describes that behavior.

Validation on Python 3.11.15 / PyTorch 2.6.0 / IPython 9.17.1:

• Six new regression cases fail before the fix and pass afterward, covering available modules, missing optional dependencies, relative imports and deprecated redirects. New and related existing tests: 16 passed, including the HyperPyYAML deprecated-import test.
• Reproduced the original error in a real IPython shell. After the fix, %autoreload 2; import speechbrain succeeds without loading NLP/Flair. Editing a temporary module updates both its lazy export and an already-referenced function through IPython autoreload.
• Baseline/current comparisons preserve inspect, importlib reload and exported-function pickle behavior. pre-commit run -a, changed-file hooks and git diff --check pass.

No models or datasets were downloaded. The full repository suite, GPU execution and pretrained inference were not run.

Implemented and tested autonomously with OpenAI Codex, with independent read-only review by another Codex agent.