### 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/RobertaTokenizer.java
public static final String UNKNOWN_TOKEN = "<unk>";
    public static final String SEPARATOR_TOKEN = "</s>";
    public static final String PAD_TOKEN = "<pad>";
    public static final String CLASS_TOKEN = "<s>";
    public static final String MASK_TOKEN = "<mask>";
    private static final Set<String> NEVER_SPLIT = Set.of(MASK_TOKEN);
    private final BpeAnalyzer bpeAnalyzer;
    protected final List<String> originalVocab;
    
    private final SortedMap<String, Integer> vocab;
    protected final boolean withSpecialTokens;
    protected final int sepTokenId;
    private final int clsTokenId;
    protected final int padTokenId;
    private final int maxSequenceLength;
    protected RobertaTokenizer(
        List<String> originalVocab,