loss reduction for `Llama2ForCausalLM.forward`
### Feature request

In `forward` method, it outputs `loss` when `labels` are provided. But the `loss` shape is always `(1,)` because `reduction='mean'` in CrossEntropy. I wonder if I could pass `reduction='none'` and get a `(batch_size,)` shaped loss tensor.

https://github.com/huggingface/transformers/blob/641adca55832ed9c5648f54dcd8926d67d3511db/src/transformers/models/llama/modeling_llama.py#L837

### Motivation

I'm using this loss for reward-based learning.

### Your contribution

I could make a PR if needed.