feat: initial vllm-npu-plugin for Ascend NPU adaptation

- NPUPlatform: device management, HCCL process group, config adaptation
- AscendAttentionBackend: npu_fusion_attention (prefill) + npu_incre_flash_attention (decode)
- NPUCommunicator: HCCL-based distributed communication
- NPUWorker: NPU device init, memory profiling
- Custom ops: SiluAndMul, RMS norm, rotary embedding
- Plugin registered via vllm.platform_plugins entry point

Based on vllm-ascend official pattern, targeting Ascend 910B

vllm_npu/__init__.py

@@ -0,0 +1,12 @@
+"""
+vllm_npu — Ascend NPU platform plugin for vLLM.
+
+The ``register()`` function is discovered by vLLM through the
+``vllm.platform_plugins`` entry-point and returns the fully-qualified
+class name of the platform implementation.
+"""
+
+
+def register():
+    """Return the fully-qualified name of the NPU platform class."""
+    return "vllm_npu.platform.NPUPlatform"