[ExecuTorch][Core AI][1/x] Graph pass: int64/float64 dtype narrowing

[metascroy]

Adds NarrowToCoreAIDtypesPass, which casts int64/float64 graph inputs to 32-bit at the boundary and widens 64-bit outputs back. This lets index style inputs such as token ids be delegated, since Core AI supports only up to 32-bit dtypes. The model's external I/O dtypes are preserved. Includes unit tests.

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