Deprecate sequence from physical mesh axes

[NuojCheng]

Description

This PR deprecates sequence mesh axis, which maxtext no longer uses.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[gobbleturk]

You should also update our documentation at sharding.md

[gobbleturk]

What about removing sequence from the model code e.g. attention.py?

[bvandermoon]

What about removing sequence from the model code e.g. attention.py?

LGTM pending this comment

[NuojCheng]

What about removing sequence from the model code e.g. attention.py?

Could you elaborate more on sequence from the model code? I don't think we have sequence explicitly mentioned in the model code, as they are governed by the logical rule. @gobbleturk