Update on "[ET Device Support] Device-aware memory planning: separate buffers per device type"

Extends memory planning to separate device tensors from CPU tensors into distinct
memory buffers. Non-CPU TensorSpecs (e.g., CUDA) are pre-assigned device-specific
mem_ids before the greedy/naive algorithm runs, ensuring they get planned into
independent memory buffers that never share space with CPU tensors.

Differential Revision: [D97447105](https://our.internmc.facebook.com/intern/diff/D97447105/)

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Author: Gasoonjia