Improve clarity for flattened graph inputs

🚀 The feature, motivation and pitch

When passing complex model inputs, such as (nested) dictionaries, it can be a little unclear on the order of flattened inputs. This can be resolved by looking at the exported IR or program dump, but these aren't easy to parse without detailed knowledge of export/ET internals.

If such a thing doesn't already exist, it would be nice to provide a way for users to easily see what the expected input format is. Maybe printing the exported function signature? This could maybe be incorporated into the higher-level export API. Or as a verbose logging option when lowering? Implementation needs thinking through. Ideally, it would be easy for a user to understand what order to pass inputs at runtime without needing to dig into verbose, low-level info.

Alternatives

No response

Additional context

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RFC (Optional)

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cc @JacobSzwejbka @angelayi @mergennachin @byjlw

Start by reviewing the higher-level export API and the existing exported IR or program dump behavior mentioned in the issue, then examine how lowering exposes verbose information. Done means users can determine the runtime order for flattened complex inputs without needing detailed export or ExecuTorch internals.