Fix abs jvp for complex inputs

[obchain]

Proposed changes

Fixes #3744.

Abs::jvp returned tangents[0] * sign(z) for complex inputs. Since |z| is real-valued, the framework dropped the imaginary part of that complex product, leaving Re(z * t) / |z| instead of the correct directional derivative

d|z| = Re(conj(z) * t) / |z| = Re(conj(sign(z)) * t)

so the imaginary contribution had the wrong sign. The fix multiplies by conj(sign(z)) and takes the real part, giving a correct (and real) tangent for complex inputs; the real path is unchanged.

Abs::vjp previously delegated to jvp. For complex inputs the vjp and jvp differ by a conjugate, so it no longer delegates — it keeps its existing, already-correct cotangent * sign(z) form (whose real/imaginary parts are the gradients w.r.t. Re(z) and Im(z)).

The bug also surfaced through ops like mx.abs(mx.fft.rfft(x)), whose jvp is now correct.

Before:

>>> z = mx.array([1+2j]); t = mx.array([0.5-1j])
>>> mx.jvp(mx.abs, [z], [t])[1][0]      # Re(z*t)/|z|, wrong
>>> mx.real(mx.conj(z)*t)/mx.abs(z)     # correct

Added test_complex_abs_grad to test_autograd.py checking the jvp against the hand-computed value, the vjp against cotangent * sign(z), and that real inputs are unaffected.

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