[bug] Fix the mapping from virtual axes to physical axes again

[strongoier]

Related issue = #3154

In Taichi docs, there is a subsection introducing row-major vs column-major dense layouts:

taichi/docs/lang/articles/advanced/layout.md

Line 87 in 801f7bd

### Row-major versus column-major

However, in Taichi v0.8.1,

ti.root.dense(ti.j, 2).dense(ti.i, 3).place(x)

is equivalent to

ti.root.dense(ti.i, 2).dense(ti.j, 3).place(x)

The reason is that virtual axes (user-view) are set according to the appearance order in SNode definition of physical axes (ti.i, ti.j, ... in the physical world). In the first example, the first axis in user-view is ti.j, and the second axis is ti.i, while in the second example, the first axis is ti.i, and the second axis is ti.j. Now comes the tricky thing - as for the memory layouts, in the first example we split ti.j first, and in the second example we split ti.i first. That is, the major axes in layouts are the same as the first axes in user-view. So essentially there is no user-viable difference between these two examples, which breaks the docs.

This PR, together with #3159, fixes this issue by setting virtual axes according to the alphabetical order of physical axes. Now in both examples, the first axis in user-view is ti.i, and the second axis in user-view is ti.j. Then the column-major layout is finally created in the first example.

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[k-ye]

LGTM! Would appreciate if this bug could be described better though..

[strongoier]

LGTM! Would appreciate if this bug could be described better though..

The description has just been updated.