### 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/ZeroShotClassificationProcessor.java
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import static org.elasticsearch.xpack.core.ml.inference.trainedmodel.InferenceConfig.DEFAULT_RESULTS_FIELD;
public class ZeroShotClassificationProcessor extends NlpTask.Processor {
    private final int entailmentPos;
    private final int contraPos;
    private final String[] labels;
    private final String hypothesisTemplate;
    private final boolean isMultiLabel;
    private final String resultsField;
    ZeroShotClassificationProcessor(NlpTokenizer tokenizer, ZeroShotClassificationConfig config) {
        super(tokenizer);
        List<String> lowerCased = config.getClassificationLabels()
            .stream()
            .map(s -> s.toLowerCase(Locale.ROOT))
            .collect(Collectors.toList());
        this.entailmentPos = lowerCased.indexOf("entailment");
        this.contraPos = lowerCased.indexOf("contradiction");
        if (entailmentPos == -1 || contraPos == -1) {
            throw ExceptionsHelper.badRequestException(
                "zero_shot_classification requires [entailment] and [contradiction] in classification_labels"
            );
        }
        this.labels = Optional.ofNullable(config.getLabels()).orElse(List.of()).toArray(String[]::new);
        this.hypothesisTemplate = config.getHypothesisTemplate();
        this.isMultiLabel = config.isMultiLabel();
        this.resultsField = config.getResultsField();
    }
    @Override
    public void validateInputs(List<String> inputs) {