Add complex arctan

[pearu]

Per https://github.com/pearu/complex_function_validation/tree/main/mpmath_jax_results_dev, JAX arctan accuracy is validated as BAD.

[pearu]

ULP difference statistics of previous implementations in StableHLO:

../stablehlo/tests/math/atan_complex128.mlir
ULP difference == 0 count is 1222
ULP difference == 1 count is 1476
ULP difference == 2 count is 12
ULP difference == 3 count is 6
ULP difference >= 4 count is 999285
../stablehlo/tests/math/atan_complex64.mlir
ULP difference == 0 count is 3326
ULP difference == 1 count is 6970
ULP difference == 2 count is 3384
ULP difference == 3 count is 3418
ULP difference >= 4 count is 984903
../stablehlo/tests/math/atan_float32.mlir
../stablehlo/tests/math/atan_float64.mlir
../stablehlo/tests/math/atanh_complex128.mlir
ULP difference == 0 count is 239712
ULP difference == 1 count is 2853
ULP difference == 2 count is 616
ULP difference == 3 count is 8
ULP difference >= 4 count is 758816
../stablehlo/tests/math/atanh_complex64.mlir
ULP difference == 0 count is 257275
ULP difference == 1 count is 24034
ULP difference == 2 count is 1280
ULP difference == 3 count is 2903
ULP difference >= 4 count is 716513
../stablehlo/tests/math/atanh_float32.mlir
ULP difference == 0 count is 987391
ULP difference == 1 count is 12610
ULP difference == 2 count is 0
ULP difference == 3 count is 0
ULP difference >= 4 count is 0
../stablehlo/tests/math/atanh_float64.mlir
ULP difference == 0 count is 996685
ULP difference == 1 count is 3316
ULP difference == 2 count is 0
ULP difference == 3 count is 0
ULP difference >= 4 count is 0

[pearu]

After using algorithms defined in FA:

../stablehlo/tests/math/atan_complex128.mlir
ULP difference == 0 count is 941281
ULP difference == 1 count is 60320
ULP difference == 2 count is 400
ULP difference == 3 count is 0
ULP difference >= 4 count is 0
../stablehlo/tests/math/atan_complex64.mlir
ULP difference == 0 count is 868953
ULP difference == 1 count is 130204
ULP difference == 2 count is 2832
ULP difference == 3 count is 12
ULP difference >= 4 count is 0
../stablehlo/tests/math/atan_float32.mlir
../stablehlo/tests/math/atan_float64.mlir
../stablehlo/tests/math/atanh_complex128.mlir
ULP difference == 0 count is 941283
ULP difference == 1 count is 60322
ULP difference == 2 count is 400
ULP difference == 3 count is 0
ULP difference >= 4 count is 0
../stablehlo/tests/math/atanh_complex64.mlir
ULP difference == 0 count is 868955
ULP difference == 1 count is 130206
ULP difference == 2 count is 2832
ULP difference == 3 count is 12
ULP difference >= 4 count is 0
../stablehlo/tests/math/atanh_float32.mlir
ULP difference == 0 count is 987391
ULP difference == 1 count is 12610
ULP difference == 2 count is 0
ULP difference == 3 count is 0
ULP difference >= 4 count is 0
../stablehlo/tests/math/atanh_float64.mlir
ULP difference == 0 count is 996685
ULP difference == 1 count is 3316
ULP difference == 2 count is 0
ULP difference == 3 count is 0
ULP difference >= 4 count is 0