Use correct type of indices in ForeachUtils.h

[cyyever]

Fix a type mismatch detected by MSVC:

C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33519\include\xutility(255): warning C4267: “初始化”: 从“size_t”转换到“_Ty”，可能丢失数据
        with
        [
            _Ty=int
        ]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.39.33519\include\xutility(255): note: 模板实例化上下文(最早的实例化上下文)为
pytorch/aten/src\ATen/native/ForeachUtils.h(363): note: 查看对正在编译的函数 模板 实例化“_Ty &std::vector<_Ty,std::allocator<_Ty>>::emplace_back<const I&>(const I &)”的引用
        with
        [
            _Ty=int,
            I=size_t
        ]

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[cyyever]

@pytorchmergebot merge

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