Bugs when using zigzag_ring_flash_attn: RuntimeError: Number of requests do not match number of collectives

[WeixuanXiong]

您好，我在使用EasyContext的zigzag_ring_flash_attn模式的时候报错如上 我的所有数据都被group by length到32768+1的长度上（根据https://github.com/jzhang38/EasyContext/issues/31#issue-2308064466）

在数据并行模式下可以正常运行，但序列并行报错。

code:

def main(args):
    if args.output_dir:
        os.makedirs(args.output_dir, exist_ok=True)
    if args.wandb:
        import wandb

        wandb.login()
    set_seed(args.seed)

    timeout = InitProcessGroupKwargs(timeout=timedelta(seconds=1_000_000))

    accelerator = Accelerator(
        gradient_accumulation_steps=args.gradient_accumulate_every,
        mixed_precision="bf16",
        log_with="wandb" if args.wandb else None,
        kwargs_handlers=[timeout],
        # fsdp_plugin=fsdp_plugin,
    )
    accelerator.init_trackers(project_name=args.wandb, init_kwargs={"wandb":{"name":args.output_dir.split("/")[-1]}})
    accelerator.print(f"Total GPUS: {accelerator.num_processes}")
    
    model = AutoModelForCausalLM.from_pretrained(
        args.model,
        device_map=accelerator.device,
        torch_dtype=torch.bfloat16,
        rope_theta=args.rope_theta,
        _attn_implementation="flash_attention_2",
    )
    
#     tokenizer = AutoTokenizer.from_pretrained(
#         args.model,
#         trust_remote_code=True,
#         # llama不支持fast
#     )
    try:
        train_dataset = load_dataset(args.dataset)
    except:
        train_dataset = load_from_disk(args.dataset)
    if isinstance(train_dataset, DatasetDict):
        train_dataset = train_dataset["train"]
#     train_dataset = QwenSFTDataset(args.dataset, tokenizer, args)

    assert isinstance(
        model, (transformers.LlamaForCausalLM, transformers.MistralForCausalLM)
    ), "Only support llama and mistral model"
    model_type = (
        "llama" if isinstance(model, transformers.LlamaForCausalLM) else "mistral"
    )
    apply_seq_parallel_monkey_patch(args.parallel_mode, model_type)

    if "input_ids" not in train_dataset.column_names:
        raise RuntimeError("Dataset must include an `input_ids` feature")
    # remove everything that is not input_ids
    to_remove = [col for col in train_dataset.column_names if col != "input_ids"]
    train_dataset = train_dataset.remove_columns(to_remove)
    train_dataset = train_dataset.shuffle(seed=args.seed)
    print("Dataset Size:", len(train_dataset))
    train_loader = DataLoader(
        train_dataset,
        collate_fn=default_data_collator,
        shuffle=True,
        batch_size=args.batch_size,
    )
    if args.learning_rate != 2e-5:
        accelerator.print(f"Warning: You also need to modify accelerate_configs/zero3_offload.json to change the learning rate")
    optim = DummyOptim(model.parameters(), lr=args.learning_rate)
    scheduler = DummyScheduler(
        optim,
        num_training_steps=args.max_train_steps,
        total_num_steps=args.max_train_steps,
    )
    model, optim, scheduler = accelerator.prepare(model, optim, scheduler)
    train_loader = prepare_dataloader(args.parallel_mode, train_loader, accelerator)
    model.gradient_checkpointing_enable()

    accelerator.register_for_checkpointing(scheduler)

    accelerator.print(f"Max train steps: {args.max_train_steps}")
    progress_bar = tqdm(
        range(args.max_train_steps), disable=not accelerator.is_local_main_process
    )
    completed_steps = 0

    model.train()
    loss_func = CrossEntropyLoss(inplace_backward=True)
    for step, batch in enumerate(train_loader):
        input_ids = batch["input_ids"][..., : args.seq_length + 1][..., :-1]
        target_ids = batch["input_ids"][..., : args.seq_length + 1][..., 1:]
        position_ids = (
            torch.arange(args.seq_length).unsqueeze(0).expand(input_ids.shape[0], -1)
        )
        # shard the input_ids according to the world size and rank according to zig zag attention
        # print(input_ids.shape, position_ids.shape) # these values must be equal
        
        prepared = prepare_seq_parallel_inputs(
            args.parallel_mode,
            input_ids,
            position_ids,
            target_ids,
            accelerator.process_index,
            accelerator.num_processes,
            accelerator.device,
        )
        local_input_ids = prepared["local_input_ids"]
        local_position_ids = prepared["local_position_ids"]
        local_target_ids = prepared["local_target_ids"]

        loss_log = None
        with accelerator.accumulate(model):
            logits = model(
                local_input_ids,
                position_ids=local_position_ids,
            ).logits
            loss = loss_func(
                logits.reshape(-1, logits.shape[-1]), local_target_ids.reshape(-1)
            )
            accelerator.backward(loss)

            if accelerator.sync_gradients:
                # pay attention here. When any seq parallel algo is turned on. This technically only log the very first chunk's loss
                # and what is the first chunk really depends on how do you shard the sequence
                # for zig zag attention, the first chunk contains the left most and rightmost tokens
                # so you cannot compare the (logged) loss of dist attention and zigzag ring attention.
                # loss_log = {"loss": loss.item(), "ppl": math.exp(loss.item())}

                # we now try gathered loss to verify if ring attention and dist flash attention produce the same loss
                # this may slow down the training
                gathered_loss = accelerator.reduce(loss.clone().detach(), "mean")
                loss_log = {
                    "loss": gathered_loss.item(),
                    "ppl": math.exp(gathered_loss.item()),
                }
                accelerator.log(loss_log, step=completed_steps)

            optim.step()
            scheduler.step()
            optim.zero_grad()

        if accelerator.sync_gradients:
            progress_bar.update(1)
            if loss_log is not None:
                progress_bar.set_postfix(loss_log)
            completed_steps += 1

        if completed_steps >= args.max_train_steps:
            break

    accelerator.print(f"Training Finished")
    accelerator.end_training()

    if args.output_dir is not None:
        accelerator.print(f"Saving model to {args.output_dir}")

        accelerator.wait_for_everyone()

        state_dict = accelerator.get_state_dict(model)

        accelerator.unwrap_model(model).save_pretrained(
            f"{args.output_dir}",
            is_main_process=accelerator.is_main_process,
            save_function=accelerator.save,
            state_dict=state_dict,
        )

        accelerator.print(f"Saving Finished")