get [gpu_id, its_free_ram] for the first gpu with highest available RAM

def gpu_with_max_free_mem():
    "get [gpu_id, its_free_ram] for the first gpu with highest available RAM"
    mem_all = gpu_mem_get_all()
    if not len(mem_all): return None, 0
    free_all = np.array([x.free for x in mem_all])
    id = np.argmax(free_all)
    return id, free_all[id]