Function dispatch_copy_f32 

pub fn dispatch_copy_f32(
    encoder: &mut CommandEncoder,
    registry: &mut KernelRegistry,
    device: &DeviceRef,
    src: &MlxBuffer,
    dst: &MlxBuffer,
    src_offset: usize,
    dst_offset: usize,
    count: usize,
) -> Result<()>

Copy count f32 elements from src[src_offset..] to dst[dst_offset..].

Used during prefill to scatter/gather rows between large prefill buffers and single-token activation buffers.