fix(median): handle empty axes in mlx_median_axes

[aki-xavier]

Summary

mlx_median_axes(res, a, NULL, 0, keepdims, s) — the "reduce over all dimensions" spelling — builds an empty axes vector that flows into the axes overload of mlx::core::median. That overload moves all axes to the back and calls flatten(start_axis == ndim), which is rejected by the assertion:

[flatten] start_axis must be less than or equal to end_axis

This makes the all-axes median unusable through the _axes entry point. The dedicated no-axis entry point mlx_median already routes to the full-reduce overload and works; only the _axes path is broken.

Reproduction

mlx_array res = mlx_array_new();
float data[4] = {1, 2, 3, 4};
int shape[2] = {2, 2};
mlx_array a = mlx_array_new_data(data, shape, 2, MLX_FLOAT32);

int rc = mlx_median_axes(&res, a, NULL, 0, false, mlx_default_cpu_stream_new());
// Before: rc == 1, prints "[flatten] start_axis must be less than or equal to end_axis"
// After:  rc == 0, res == 2.5

Fix

When axes_num == 0, route to the full-reduce overload mlx::core::median(a, keepdims, s) directly, mirroring the dedicated mlx_median entry point. The normal axes_num > 0 path is unchanged.

Verification

Built mlxc (CPU backend) and ran a small program against it:

case	before	after
mlx_median(a, keepdims=false)	rc=0, 2.5	rc=0, 2.5
mlx_median_axes(a, NULL, 0, ...)	rc=1, flatten assert	rc=0, 2.5
mlx_median_axes(a, axis=0, ...)	rc=0	rc=0

Fixes #128.