### Description

When completing a zero_shot classification task, there is the input sequence (provided by the user), and labels (also provided by the user). 

Tokenization as it is done now tokenizes the input sequence len(labels) times. We should really only do it once and pass those tokenized values to request builder. 

Some of the changes introduce here: https://github.com/elastic/elasticsearch/pull/84777 should make implementing this simpler (as there will be a unified temporary tokenization result object).
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/nlp/tokenizers/BertTokenizer.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.ml.inference.nlp.tokenizers;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;
import org.elasticsearch.common.util.set.Sets;
import org.elasticsearch.xpack.core.ml.inference.trainedmodel.Tokenization;
import org.elasticsearch.xpack.core.ml.utils.ExceptionsHelper;
import org.elasticsearch.xpack.ml.inference.nlp.NlpTask;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.OptionalInt;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;