Fix Gemma2 synced multi-GPU generation
Author: ManukyanD
Pull request: yes

# What does this PR do?

Generation with Gemma2ForCausalLM in synced multi-GPU settings crashes because the cache_position goes out of bounds. This PR addresses the issue.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.


@ArthurZucker
@gante


Comments:
@gante Couldn't you, please, review this PR?

@ArthurZucker Sure, here is an example script
```
from transformers import (
    GenerationConfig,
    Seq2SeqTrainingArguments,
    Seq2SeqTrainer,
    AutoTokenizer,
    Gemma2ForCausalLM,
)

from datasets import load_dataset


model_id = "google/gemma-2-2b-it"
tokenizer = AutoTokenizer.from_pretrained(model_id)
model = Gemma2ForCausalLM.from_pretrained(model_id)

dataset = load_dataset("google/boolq")

prompt = """
Answer the question according to the provided passage.

Passage:
{passage}

Question:
{question}
"""


def collator(examples):

    inputs = [
        [
            {
                "role": "user",
                "content": prompt.format(
                    passage=example["passage"], question=example["question"]
                ),
            }
        ]
        for example in examples
    ]
    inputs = tokenizer.apply_chat_template(
        inputs, add_generation_prompt=True, tokenize=False
    )
    return tokenizer(
        inputs, padding=True, add_special_tokens=False, return_tensors="pt"
    )


def compute_metrics(pred):
    # compute some metrics
    return {"score": 0.5}


training_args = Seq2SeqTrainingArguments(
    output_dir="./output",
    generation_config=GenerationConfig(
        pad_token_id=tokenizer.pad_token_id,
        bos_token_id=tokenizer.bos_token_id,
        eos_token_id=tokenizer.eos_token_id,
        max_new_tokens=512,
        use_cache=True,
    ),
    remove_unused_columns=False,
    predict_with_generate=True,
    per_device_eval_batch_size=8,
    deepspeed="./deepspeed_config.json",
)
trainer = Seq2SeqTrainer(
    model=model,
    tokenizer=tokenizer,
    args=training_args,
    data_collator=collator,
    compute_metrics=compute_metrics,
    train_dataset=dataset["train"],
    eval_dataset=dataset["validation"],
)
trainer.evaluate()
```
I am running this with deepspeed. The following is the deepspeed config
```
{
    "zero_optimization": {
        "stage": 3,
        "overlap_comm": true,
        "contiguous_gradients": true,
        "sub_group_size": 1e9,
        "stage3_max_live_parameters": 1e9,
        "stage3_max_reuse_distance": 1e9,
        "stage3_gather_16bit_weights_on_model_save": true
    },
    "train_batch_size": "auto",
    "train_micro_batch_size_per_gpu": "auto",
    "gradient_accumulation_steps": "auto"
}
```

@ArthurZucker let's merge this and a related PR (https://github.com/huggingface/transformers/pull/35893), seems like a real issue and has been reported by another user recently

@ManukyanD thank you for the fix 💛 

cc @SunMarc: this PR (and #35893) copies the fix in #34095 into functions that are overwritten in specific models. There, you [mentioned](https://github.com/huggingface/transformers/pull/34095#issuecomment-2411047676) we had no tests for it [multigpu + generate] -- any chance you had a look at it, or would you be able to share pointers to add an appropriate lightweight test? 🙏