[ML] fix NLP question_answering task when best answer is only one token (elastic#88347)

There are scenarios when question_answering find the best start/end token and they are the same token. An example of this is:

context: "My name is Ben and I live in London" question: "Where do I live?"

The correct answer here is London and its a single token. Without this fix, we will return in London with a lower probability.

Author: benwtrent

docs/changelog/88347.yaml

@@ -0,0 +1,5 @@
+pr: 88347
+summary: Fix NLP `question_answering` task when best answer is only one token
+area: Machine Learning
+type: bug
+issues: []

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/nlp/QuestionAnsweringProcessor.java

@@ -212,7 +212,7 @@ static void topScores(
                 if (startNormalized[i] == 0) {
                     continue;
                 }
-                for (int j = i + 1; j < (maxAnswerLength + i) && j < tokenSize; j++) {
+                for (int j = i; j < (maxAnswerLength + i) && j < tokenSize; j++) {
                     double score = startNormalized[i] * endNormalized[j];
                     if (score > maxScore) {
                         maxScore = score;
@@ -224,7 +224,7 @@ static void topScores(
             return;
         }
         for (int i = seq2Start; i < tokenSize; i++) {
-            for (int j = i + 1; j < (maxAnswerLength + i) && j < tokenSize; j++) {
+            for (int j = i; j < (maxAnswerLength + i) && j < tokenSize; j++) {
                 topScoresCollector.accept(
                     new ScoreAndIndices(i - seq2Start, j - seq2Start, startNormalized[i] * endNormalized[j], spanIndex)
                 );