[ML] Fix Array out of bounds exception in the XLM Roberta tokenizer

[davidkyle]

The following ArrayIndexOutOfBoundsException has been observed tokenising certain Unicode characters in the XLM Roberta tokenizer. The overflow comes from a fixed sized reusable buffer that holds the normalised form of a single unicode character encoded as UTF-8. The normalised form can be much longer than the allocated buffer size (8 chars), for example 'ﷺ' normalises to 33 characters. That is surprising so I compared the output to other SentencePiece implementations and the results are in agreement.

The fix here is to spend a few bytes increasing the buffer size.

java.lang.ArrayIndexOutOfBoundsException: Index 8 out of bounds for length 8
	at org.apache.lucene.util.UnicodeUtil.UTF8toUTF16(UnicodeUtil.java:650) ~[lucene-core-9.9.2.jar:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.PrecompiledCharMapNormalizer.normalize(PrecompiledCharMapNormalizer.java:194) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.PrecompiledCharMapNormalizer.fill(PrecompiledCharMapNormalizer.java:263) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.PrecompiledCharMapNormalizer.read(PrecompiledCharMapNormalizer.java:242) ~[?:?]
	at org.apache.lucene.analysis.CharacterUtils.readFully(CharacterUtils.java:183) ~[lucene-core-9.9.2.jar:?]
	at org.apache.lucene.analysis.CharacterUtils.fill(CharacterUtils.java:159) ~[lucene-core-9.9.2.jar:?]
	at org.apache.lucene.analysis.CharacterUtils.fill(CharacterUtils.java:177) ~[lucene-core-9.9.2.jar:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.UnigramTokenizer$SimpleWhitespaceTokenizer.next(UnigramTokenizer.java:464) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.UnigramTokenizer.incrementToken(UnigramTokenizer.java:165) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.XLMRobertaTokenizer.innerTokenize(XLMRobertaTokenizer.java:173) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.NlpTokenizer.tokenize(NlpTokenizer.java:60) ~[?:?]
	at org.elasticsearch.xpack.ml.inference.nlp.tokenizers.XLMRobertaTokenizer.lambda$requestBuilder$0(XLMRobertaTokenizer.java:132) ~[?:?]

This PR also removes some unused fields.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

Hi @davidkyle, I've created a changelog YAML for you.

[droberts195]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.13