fix(group_offloading): support Ascend NPU stream when available

[li-lizhe]

apply_group_offloading with use_stream=True only created a stream for CUDA (via torch.cuda.Stream()) or Intel XPU (via torch.Stream()), and raised Using streams for data transfer requires a CUDA device, or an Intel XPU device. on every other accelerator.

On Ascend NPU, torch.npu.Stream() is available (exposed by torch_npu), so stream-based onload/offload should be supported there too.

Changes:

• Add an npu branch consistent with the existing CUDA/XPU checks (hasattr(torch, "npu") and torch.npu.is_available() -> torch.npu.Stream())
• Update the error message to mention the Ascend NPU device

Testing:
Verified on an Ascend 910B NPU (torch 2.14 + torch_npu):

• Old code raises ValueError (confirmed torch.cuda.is_available()=False, torch.xpu.is_available()=False)
• New code: torch.npu.Stream() constructs successfully
• hasattr(torch, "npu")=True, torch.npu.is_available()=True

5 lines added — minimal, device-agnostic, consistent with existing accelerator dispatch pattern.

[github-actions]

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