[ET Device Support] Add NonConstBufferDevice schema for per-buffer device mapping

[Gasoonjia]

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Adds the NonConstBufferDevice table to the FlatBuffer schema (program.fbs) and the
corresponding Python dataclass to schema.py. This enables mapping each non-constant
planned memory buffer to a specific device type (CPU, CUDA, etc.).

The field is optional and absent for CPU-only programs, ensuring zero binary size regression.

Differential Revision: D97335597

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 621098c with merge base d306410 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• periodic / test-models-linux (cmake, mv3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 1

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