Fix sign and float64 precision of portable remainder

[usmanmateen]

Summary

Fixes #23682.

ATen's remainder gives a result with the sign of the divisor. utils::remainder_override in kernels/portable/cpu/util/math_util.h follows that for floating-point types: it adds b when exactly one operand is negative and the fmod result is nonzero, as the comment above the overloads describes. The integer overload returned a % b, so -7 % 3 gave -1 instead of 2. This adds the same correction for signed integer types.

The floating-point overload also kept the fmod result in a float, so float64 results were rounded to float precision (123456789.123456789 % 1000 gave 789.1234741210938 instead of 789.1234567910433). It now uses CTYPE.

Both remainder.Tensor_out and remainder.Scalar_out in the portable kernels use this helper, and so does the Cadence HiFi remainder operator.

Test plan

Added two tests to kernels/test/op_remainder_test.cpp:

• IntegerResultHasSignOfDivisor covers all four sign combinations plus exact multiples, for both the Tensor and Scalar variants.
• DoubleKeepsDoublePrecision checks a float64 case with atol=1e-6. Rounding to float would be off by about 1.7e-5.

I built portable_kernels_test with CMake (EXECUTORCH_BUILD_TESTS=ON) on Linux x86_64 (WSL2). Before this change, both new tests fail; after it, all three OpRemainderOutTest tests pass, and the full portable_kernels_test run passes (1650 tests). clang-format 18.1.3 reports no changes on the two files.

This PR was authored with AI assistance (Claude Code).

cc @nil-is-all

[pytorch-bot]

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[usmanmateen]

@pytorchbot label "release notes: ops & kernels"