Route packed sdpa rows through torch.nn.attention.varlen.varlen_attn
Author: winglian
Pull request: yes

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# What does this PR do?

PyTorch recently added varlen support to SDPA. Upstreaming this from axolotl to transformers. 

see https://docs.pytorch.org/tutorials/intermediate/variable_length_attention_tutorial.html


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### CI recap

**Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=47210)
**Latest run:** [29023370645](https://github.com/huggingface/transformers/actions/runs/29023370645)
**Result:** `failure` | Grafana metrics are not available yet.

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cc @arthurzucker!