argmin-negation-overflow: fix aten.min.dim indices at dtype-extremal minima

[arames]

Symptom

aten.min.dim returns the wrong indices when the true minimum is a
dtype-extremal integer value (e.g. uint8 min 0, int8 min -128). values
is correct; only indices is wrong. Silent — no crash.

Root cause

replace_min_dim in coreai_torch/_aten_to_core.py computed argmin as
argmax(x * -1). Negation via multiply is not order-reversing at integer
extremes: 0 * -1 == 0 (not the range top) for unsigned, and -128 * -1
overflows int8. So argmax scores the true minimum incorrectly.

Fix

Reverse order for integer dtypes with the bitwise complement ~x = x ^ -1
(broadcasting_bitwise_xor with an all-ones constant) instead of negation.
~x = -x - 1 is a strictly decreasing bijection over the full range of every
integer dtype (signed and unsigned), never overflows, and introduces no new
ties, so argmax(~x) == argmin(x) with first-index-on-tie semantics preserved.
The float path keeps x * -1 (exact, no overflow).

Tests

Adds dtype-extremal coverage (uint8 min 0; int8/int16/int32 dtype-min),
duplicate-minima tie-break, and a float case proving the float path is
unchanged. Reverting the fix fails all integer cases.

[gokulkrishna98]

Please install pre-commit, to resolve linting issues: https://github.com/apple/coreai-torch/blob/main/.pre-commit-config.yaml

[arames]

Installed pre-commit and re-ran it over the changed files (ruff check, ruff format, plus the generic hooks and license-header check) — all clean now. Squashed the branch down to a single commit as well.

[gokulkrishna98]

Hi Alex, for merging, your commits must have verified signature. I think you have to setup signing-key ssh for this GitHub account.