Add tip to tree reduce and reduce_associative about how to exclude leaves from the reduction.

[carlosgmartin]

Adds a tip to tree.reduce and tree.reduce_associative about how to exclude leaves from the reduction.

Context: #30156.

[carlosgmartin]

@jakevdp How does this look? If you prefer a different format, let me know.

[jakevdp]

Thanks - sorry I lost track of this.

The tip is a bit abstract, and I'm not entirely sure in what contexts it might be useful. Can you say more? Maybe including the tip in Examples rather than in Notes would be more helpful?

[carlosgmartin]

For me, personally, this came up in the context of this PR: I wanted to exclude zero-size leafs when computing the min/max of a whole pytree.

This pattern can be helpful whenever users want to exclude certain leafs from a reduction, for any reason.

To wit, I was originally going to request a filter argument to tree.reduce, but then realized it can be accomplished by simply converting (via tree.map) leafs to something that isn't considered a leaf by tree.reduce (and thus gets excluded from the reduction), e.g. None.

If you think another wording would be clearer, I can reword it.