CANN: Refactor ND to NZ workspace to be per-device (llama/15763)

CANN: Refactor ND to NZ workspace to be per-device (llama/15763)

* CANN:Refactor ND to NZ workspace to be per-device in Ascend backend

- Replaced the previous single global ND→NZ workspace with a per-device
  cache using unordered_map keyed by device ID.
- Functions `release_nz_workspace`, `relloc_nz_workspace`, and
  `get_nz_workspace` now manage workspace independently for each device,
  preventing memory conflicts in multi-device / pipeline parallel scenarios.
- This change fixes potential precision issues caused by workspace
  overwrites when multiple devices perform ND→NZ conversions concurrently.

Co-authored-by: hipudding <redacted>
* refactor

Signed-off-by: noemotiovon <redacted>
* rename

Signed-off-by: noemotiovon <redacted>
* fix review comments

Signed-off-by: noemotiovon <redacted>
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Signed-off-by: noemotiovon <redacted>
Co-authored-by: hipudding <redacted>