[Ascend] Integrate transformer_engine_npu && Fix the GEMM operator bug in the reference backend

[Darryl233]

Summary

This PR adds Ascend NPU support to the TE-FL plugin system through torch_npu and transformer_engine_npu, and fixes backward-path issues in the reference GEMM implementation.

Changes

Ascend NPU backend

• Add automatic NPU availability detection and vendor-priority registration.
• Add support for:
  • FlashAttention with SBHD, BSHD, and THD layouts
  • RMSNorm forward and backward
  • Generic and grouped GEMM
  • Multi-tensor scale and L2-norm operations
• Add THD ↔ BSHD conversion operators.
• Keep NPU dependencies lazily imported.

Reference GEMM fixes

• Fix output shape restoration for transposed inputs.
• Do not add forward bias in backward mode.
• Compute fused bias gradients.
• Apply dGeLU using the saved forward activation.
• Preserve correct alpha scaling and 3D input behavior.

Testing

Added coverage for:

• FlashAttention forward/backward accuracy and causal masking
• RMSNorm forward/backward
• Generic and grouped GEMM
• Multi-tensor and FP8 scale operations
• Reference GEMM backward behavior

Verified on Ascend 910C:

46 passed

Deps

It depends on TransformerEngineNPU. The package natively generated by TransformerEngineNPU is named transformer_engine. Relevant packaging logic needs to be modified so that the generated package is named transformer_engine_npu.

[CLAassistant]

All committers have signed the CLA.

[lxd-cumt]

LGTM