[misc] move mxfp_compat into device to decouple from quantization init chain (#6918)

### What this PR does / why we need it?
`mxfp_compat` only provides dtype/symbol compatibility helpers for
different `torch_npu` versions, but it was placed under
`vllm_ascend.quantization`. Importing it from device/ops paths could
trigger `quantization/__init__.py` and pull in heavy quantization method
dependencies, increasing startup coupling and causing import-cycle risk
(especially on 310P paths).

### Does this PR introduce _any_ user-facing change?
No functional behavior change intended.

### How was this patch tested?
CI passed.

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

---------

Signed-off-by: linfeng-yuan <1102311262@qq.com>

vllm_ascend/device/mxfp_compat.py

@@ -0,0 +1,43 @@
+import torch
+import torch_npu
+
+# TODO(linfeng): Temporary compatibility shim for MXFP4/MXFP8 because current torch_npu
+# releases do not expose the required dtype attributes yet. Simplify or remove this
+# file after the torch_npu release in March 2026 includes those dtype symbols.
+FLOAT8_E8M0FNU_DTYPE = getattr(torch_npu, "float8_e8m0fnu", getattr(torch, "float8_e8m0fnu", None))
+FLOAT4_E2M1FN_X2_DTYPE = getattr(torch_npu, "float4_e2m1fn_x2", getattr(torch, "float4_e2m1fn_x2", None))
+HIFLOAT8_DTYPE = getattr(torch_npu, "hifloat8", None)
+
+
+def _get_missing_symbols(symbols: tuple[str, ...]) -> list[str]:
+    return [symbol for symbol in symbols if not hasattr(torch_npu, symbol)]
+
+
+def _ensure_symbols_available(feature: str, symbols: tuple[str, ...]) -> None:
+    missing_symbols = _get_missing_symbols(symbols)
+    if not missing_symbols:
+        return
+    missing_symbols_str = ", ".join(missing_symbols)
+    raise RuntimeError(
+        f"{feature} requires a newer torch_npu runtime. Missing symbols: {missing_symbols_str}. "
+        "Please upgrade torch_npu or disable MXFP quantization."
+    )
+
+
+def ensure_mxfp8_scale_dtype_available(feature: str) -> None:
+    _ensure_symbols_available(feature, ("float8_e8m0fnu",))
+
+
+def ensure_mxfp4_dtype_available(feature: str) -> None:
+    _ensure_symbols_available(feature, ("float4_e2m1fn_x2", "float8_e8m0fnu"))
+
+
+def ensure_mxfp8_linear_available(feature: str) -> None:
+    _ensure_symbols_available(feature, ("float8_e8m0fnu", "npu_dynamic_mx_quant", "npu_quant_matmul"))
+
+
+def ensure_mxfp8_moe_available(feature: str) -> None:
+    _ensure_symbols_available(
+        feature,
+        ("float8_e8m0fnu", "npu_dynamic_mx_quant", "npu_grouped_matmul_swiglu_quant_v2"),
+    )