### Elasticsearch Version

8.9.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

any

### Problem Description

If the context text supplied to the Question Answering task is longer than the model's `max_sequence_length` and `truncate` is set to `none` then inference on the model fails with the message:

```
status_exception: question answering result has invalid dimension, expected 2 found [6] 
```
<img width="1438" alt="Screenshot 2023-07-25 at 08 21 37" src="https://github.com/elastic/elasticsearch/assets/2353640/12d296f6-03b2-4cda-89ce-7399348c4104">


### Steps to Reproduce

Use long input text to the question answering task


### Work Aroundf
The fix is to truncate the context which unfortunately may remove the part of the document containing the answer. For reference the call is documented here

```
POST _ml/trained_models/distilbert-base-cased-distilled-squad/deployment/_infer
{
  "docs":[{ "text_field": "question answering context"}],
  "inference_config": {
    "question_answering": {
      "question": "The question to ask",
      "tokenization": {
        "bert": {
          "truncate": "second",
          "span": -1          
        }
      }
    }
  }
}
````

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/nlp/QuestionAnsweringProcessor.java
double[] startNormalized = normalizeWith(start, i -> {
            if (i == 0) {
                return true;
            }
            return i >= seq2Start && i < tokenSize - 1;
        }, -10000.0);
        double[] endNormalized = normalizeWith(end, i -> {
            if (i == 0) {
                return true;
            }
            return i >= seq2Start && i < tokenSize - 1;
        }, -10000.0);
        
        endNormalized[0] = startNormalized[0] = 0.0;
        if (numAnswersToGather == 1) {
            ScoreAndIndices toReturn = new ScoreAndIndices(0, 0, 0.0, spanIndex);
            double maxScore = 0.0;
            for (int i = seq2Start; i < tokenSize; i++) {
                if (startNormalized[i] == 0) {
                    continue;
                }
                for (int j = i; j < (maxAnswerLength + i) && j < tokenSize; j++) {
                    double score = startNormalized[i] * endNormalized[j];
                    if (score > maxScore) {
                        maxScore = score;
                        toReturn = new ScoreAndIndices(i - seq2Start, j - seq2Start, score, spanIndex);
                    }
                }
            }