Update PyTorch export tutorial (openxla#2617)

This PR updates the "Exporting StableHLO from PyTorch" tutorial.
Specifically:

- Update dependencies, combine the tensorflow dependency in the first
section to avoid warnings (about using tensorflow-cpu) on Kaggle
- Minor updates to narration and links, along with an introduction
statement
- Removed dynamic batch size code, addedd notes instead

(Supersedes openxla#2616, same updates but we get a rich diff here)

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Signed-off-by: Pavithra Eswaramoorthy <[email protected]>