Update _combining.py changing examples to use optax.multi_transform, not optax.partition

[ywrt]

Change mention of optax.partition to optax.multi_transform as optax.partition no longer exists.

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[rdyro]

This looks great!

Could you also update:

• optax/transforms/_combining_test.py to use multi_transform
• optax/transforms/__init__.py to export multi_transform in addition to the old partition (essentially adding multi_transform = partition and adding multi_transform to __all__)

Thank you!

[ywrt]

I fully renamed partition to multi_transform, and added partition/PartitionState aliases for backward compatibility.

[rdyro]

It looks great! Just a minor comment and let's merge this, thank you!