Wrong ID generation for annotations

```
{
"py/object": "ft.onto.base_ontology.EntityMention",
"py/state": {
"_embedding": [],
"_span": {
"begin": 24,
"end": 31,
"py/object": "forte.data.span.Span"
},
"_tid": 2,
"ner_type": null
}
}
```

When new annotation arrives, the _tid generated by the backend should be an int number but not a HASH string. The int number should be "the biggest id in the doc + 1"

No file or test is named. Start by locating the backend path that creates annotations and assigns _tid, then inspect how existing document annotation IDs are read; done means a new annotation receives the greatest existing document ID plus one as an integer rather than a hash string, with coverage for an existing-ID document.

Written by the indexing model from the issue text.