Move number of split quantiles from hardwired to node class member

[EgorKraevTransferwise]

Context

Just wanted to make this change before I forget about it, related to cerlymarco/linear-tree#41

Checklist

• [x ] Change meets or does not compromise the Baseline Security Requirements

[EgorKraevTransferwise]

Merging can wait until we have some actually useful changes to deploy as well :)

[wise-github-bot-app]

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