[Cpp API Compatibility] Align ScalarType related APIs

[youge325]

PR Category

Execute Infrastructure

PR Types

New features

Description

拆分自 #78707

为 C++ 兼容层（compat）补充 PyTorch 对齐的 ScalarType 辅助工具函数与量化类型支持：

• 在 c10/core/ScalarType.h 中新增：
  • isBitsType / isBarebonesUnsignedType：位宽类型与裸无符号类型判断
  • toQIntType：将基础类型映射为对应的量化类型
  • isUnderlying / toRealValueType / toComplexType：类型转换与校验
  • canCast：类型间可转换性检查（拒绝复数→实数、浮点→整数、非 Bool→Bool 等非法转换）
• 在 torch/headeronly/core/ScalarType.h 中新增：
  • isQIntType：量化整数类型判断
  • toUnderlying：量化类型到其底层类型的映射
• test/cpp/compat/c10_ScalarType_test.cc 新增单测，覆盖上述全部辅助函数的行为对齐。

是否引起精度变化

否

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[youge325]

/re-run all-failed

[SigureMo]

LGTMeow