varlen attention tutorial

[liangel-02]

This PR is a tutorial for a variable length attention API

[pytorch-bot]

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[drisspg]

@svekars any tips for reviewing this code in a more human friendly way?

[svekars]

Can you please submit this as a .py - all our tutorials are either .py or .rst and .py tutorials are converted into html and .ipynb on build. You can follow this template: https://github.com/pytorch/tutorials/blob/main/beginner_source/template_tutorial.py

[svekars]

There is more info here: https://github.com/pytorch/tutorials#contributing and there is a script that will help you convert your .ipynb to .py

[svekars]

Looking good overall. Need to add to compilers_index.rst and I added some suggestions that would help it make more discoverable on the website/google. Can also add to the What's new in the PyTorch tutorials in the index.rst.

[svekars]

There is also an error here: https://github.com/pytorch/tutorials/actions/runs/19864783846/job/56924391840?pr=3660#step:9:4582 - let me check how should we test against 2.10.