ValueError in finetuning NLLB
### System Info

- `transformers` version: 4.21.1
- Platform: Linux-5.15.0-56-generic-x86_64-with-glibc2.31
- Python version: 3.9.7
- Huggingface_hub version: 0.13.3
- PyTorch version (GPU?): 1.10.1+cu113 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no


### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

It is surprising why there is still no example of finetuning any of NLLB models (at least, the smallest one) in a huggingface transformers environment. So I have followed [this](https://huggingface.co/docs/transformers/tasks/translation) guide and adapted the code to my case, namely, `nllb-200-distilled-600M`.
My custom train and eval datasets I want to finetune `nllb-200-distilled-600M` on consist of 2 entries each, see my attached code. Running this code gives me `ValueError: You have to specify either decoder_input_ids or decoder_inputs_embeds`.

```
from transformers import AutoModelForSeq2SeqLM, NllbTokenizer, Seq2SeqTrainingArguments, Seq2SeqTrainer, DataCollatorForSeq2Seq
from datasets import Dataset
import numpy as np
import evaluate

trainPart = []
evalPart = []

def buildDataset():
    trainPart.append({'id': 0, 'translation': {
        'en': 'But this lofty plateau measured only a few fathoms, and soon we reentered Our Element.',
        'ru': 'Но это высокое плато имело размер всего в несколько саженей, и вскоре мы снова оказались в своей стихии.'}})
    trainPart.append({'id': 1, 'translation': {
        'en': 'What awakened us was a sound which sent chills of fear down my spine: the howling of the monsters\' sirens, and the reverberations of distant explosions.',
        'ru': 'Разбудили нас звуки, от которых у меня по спине побежали мурашки страха, - завывания сирен чудовищ и эхо отдаленных взрывов.'}})
    evalPart.append({'id': 0, 'translation': {
        'en': 'It could be coming from reverberations, deeper caverns caught in currents.',
        'ru': 'Это, наверное, от ревербераций в глубинных полостях, вызванных течениями.'}})
    evalPart.append({'id': 1, 'translation': {
        'en': 'There’s a four to five second reverberation.',
        'ru': 'Реверберация длится от четырех до пяти секунд.'}})


def postprocess_text(preds, labels):
    preds = [pred.strip() for pred in preds]
    labels = [[label.strip()] for label in labels]
    return preds, labels

def run():
    modelName = "nllb-200-distilled-600M"
    model = AutoModelForSeq2SeqLM.from_pretrained(modelName, use_auth_token=True)
    tokenizer = NllbTokenizer.from_pretrained(
        modelName, src_lang='eng_Latn', tgt_lang='rus_Cyrl'
    )
    trainSet = Dataset.from_list(trainPart)
    evalSet = Dataset.from_list(evalPart)

    def preprocess_function(examples):
        inputs = [example['en'] for example in examples["translation"]]
        targets = [example['ru'] for example in examples["translation"]]
        model_inputs = tokenizer(inputs, text_target=targets, max_length=128, truncation=True)
        return model_inputs

    def compute_metrics(eval_preds):
        preds, labels = eval_preds
        if isinstance(preds, tuple):
            preds = preds[0]
        decoded_preds = tokenizer.batch_decode(preds, skip_special_tokens=True)
        labels = np.where(labels != -100, labels, tokenizer.pad_token_id)
        decoded_labels = tokenizer.batch_decode(labels, skip_special_tokens=True)
        decoded_preds, decoded_labels = postprocess_text(decoded_preds, decoded_labels)
        result = metric.compute(predictions=decoded_preds, references=decoded_labels)
        result = {"bleu": result["score"]}
        prediction_lens = [np.count_nonzero(pred != tokenizer.pad_token_id) for pred in preds]
        result["gen_len"] = np.mean(prediction_lens)
        result = {k: round(v, 4) for k, v in result.items()}
        return result

    tokenized_trainset = trainSet.map(preprocess_function, batched=True)
    tokenized_evalset = evalSet.map(preprocess_function, batched=True)
    data_collator = DataCollatorForSeq2Seq(tokenizer=tokenizer, model=model)  # или modelName?
    metric = evaluate.load("sacrebleu")

    training_args = Seq2SeqTrainingArguments(
        output_dir="test_ft",
        evaluation_strategy="epoch",
        learning_rate=2e-5,
        per_device_train_batch_size=1,
        per_device_eval_batch_size=1,
        weight_decay=0.01,
        save_total_limit=3,
        num_train_epochs=2,
        predict_with_generate=True,
        fp16=True,
        push_to_hub=False,
    )

    trainer = Seq2SeqTrainer(
        model=model,
        args=training_args,
        train_dataset=tokenized_trainset,
        eval_dataset=tokenized_evalset,
        tokenizer=tokenizer,
        data_collator=data_collator,
        compute_metrics=compute_metrics,
    )

    trainer.train()


buildDataset()
run()
```


### Expected behavior

A set of finetuned model's files in my `output_dir`
You should use the [forums](https://discuss.huggingface.co/) to help debug your training code.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>There still was no help from the forums. See https://discuss.huggingface.co/t/valueerror-in-finetuning-nllb/35533