[ET][Windows] Fix ExecuTorch Windows host build portability defects

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET][Windows] Relax -Werror for ExecuTorch CPU kernels on the Windows host #20899
• -> [ET][Windows] Fix ExecuTorch Windows host build portability defects #20898

Building ExecuTorch (Vulkan backend plus core runtime) for a Windows x86_64 host with the arvr clang toolchain surfaced three genuine portability defects that fail under its strict -Werror set and generated-header wiring. These are correct fixes independent of any warning-suppression workaround.

In runtime/core/portable_type/c10/c10/targets.bzl, the arvr-mode select that supplies the generated ATen/Config.h routed every non-Android OS to ovrsource_aten_Config.h, an OVR-native-only oxx_static_library that produces no output on the Windows host. The result was fatal error: 'ATen/Config.h' file not found in every CPU kernel that includes ATen vec headers. This adds an ovr_config//os:windows branch pointing at the working generated_aten_config_header, mirroring the existing Android fallback.

In backends/vulkan/runtime/api/containers/Tensor.h, size() and dim() returned const int64_t by value; the meaningless top-level const on a scalar return trips -Werror,-Wignored-qualifiers. This header is included throughout the Vulkan backend, so it blocked vulkan_graph_runtime. The const qualifier is dropped.

In extension/data_loader/mman.h and mman_windows.cpp, #define NOMINMAX was unconditional while the toolchain already predefines it, tripping -Werror,-Wmacro-redefined when compiling mmap_data_loader (pulled in by Module). Both sites are now guarded with #ifndef NOMINMAX.

Differential Revision: D111811481

[pytorch-bot]

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