revert change to cu_seqlen_k and max_k when preparing from position_ids
Author: winglian
Pull request: yes

# What does this PR do?
follow up to PR #[39622](https://github.com/huggingface/transformers/pull/39622) as these new lines from #39474 causes this during training w multi-gpu

```
[rank2]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/flash_attn/flash_attn_interface.py", line 165, in _flash_attn_varlen_forward
[rank2]:     out, softmax_lse, S_dmask, rng_state = flash_attn_gpu.varlen_fwd(
[rank2]:                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank2]: RuntimeError: cu_seqlens_k must have shape (batch_size + 1)
```
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