### 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
QuestionAnsweringInferenceResults.TopAnswerEntry[] topAnswerList =
                new QuestionAnsweringInferenceResults.TopAnswerEntry[numAnswersToGather];
            for (int i = numAnswersToGather - 1; i >= 0; i--) {
                ScoreAndIndices scoreAndIndices = finalEntries.pop();
                TokenizationResult.Tokens tokens = tokensList.get(scoreAndIndices.spanIndex());
                int startOffset = tokens.tokens().get(1).get(scoreAndIndices.startToken).startOffset();
                int endOffset = tokens.tokens().get(1).get(scoreAndIndices.endToken).endOffset();
                topAnswerList[i] = new QuestionAnsweringInferenceResults.TopAnswerEntry(
                    tokens.input().get(1).substring(startOffset, endOffset),
                    scoreAndIndices.score(),
                    startOffset,
                    endOffset
                );
            }
            QuestionAnsweringInferenceResults.TopAnswerEntry finalAnswer = topAnswerList[0];
            return new QuestionAnsweringInferenceResults(
                finalAnswer.answer(),
                finalAnswer.startOffset(),
                finalAnswer.endOffset(),
                numTopClasses > 0 ? Arrays.asList(topAnswerList) : List.of(),
                Optional.ofNullable(resultsField).orElse(DEFAULT_RESULTS_FIELD),
                finalAnswer.score(),
                tokenization.anyTruncated()
            );
        }
    }
    /**
     *
     * @param start The starting token index scores. May include padded tokens.
     * @param end The ending token index scores. May include padded tokens.