Fix GPU ABS range

[JasonHonKL]

Proposed changes

Fix #4344

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[zcbenz]

Since this is only used once I think we should just put it in operator()(complex64_t x)

[JasonHonKL]

Hi @zcbenz gonna resolve this weekend having a trip now sorry !

[eyupcanakman]

Two more sites carry the same sqrt(re*re + im*im).
Power builds x_ln_r from it in binary_ops.h, and Sign divides by it in unary_ops.h.
Both still come back nan with your patch applied.

I put your diff on a Metal build here (M5 Pro) and ran the two values from the issue through sign and power:

gpu, complex64            #4345 alone   + the two edits              cpu
sign   (2.5e-20,-3e-20)   (nan, nan)    (0.640184, -0.768221)        (0.640184, -0.768221)
z**0.5 (2.5e-20,-3e-20)   (nan, nan)    (1.78957e-10, -8.3819e-11)   (1.78957e-10, -8.3819e-11)
z**2   (1e20, 1e20)       (nan, nan)    (-inf, inf)                  (-inf, inf)
z**2   (2.5e-20,-3e-20)   (nan, nan)    (-0, -0)                     (-2.75001e-40, -1.5e-39)

The last row stays off because the true result is subnormal and Metal flushes it, which is separate from this.

Power takes the one-line swap you already wrote, and the 0.5 falls out since complex_abs gives the magnitude rather than its square.

Sign needs a little more.
x / (complex64_t)complex_abs(x.real, x.imag) still returns nan, since operator/ in complex.h squares its own denominator, so the magnitude goes out of float32 range there instead.
Two float divisions by complex_abs(x.real, x.imag) get around it.

Either way the helper ends up with three callers across two headers, which is the argument for leaving it in utils.h.

[zcbenz]

We should add a hypot utility if we want to reuse the code:
https://github.com/pytorch/pytorch/blob/a82aae9d4a7827849ce50f31c4c7ee8f278d05f5/c10/metal/utils.h#L554