Potential REL sharding issue

I use spacy-llm for RAG, so I have integrated pretty deeply into it and 0.7.0 broke my method of prompt generation. In the process of fixing it, I've noticed something that may be a more general bug.

The REL shard reducer [unpacks RelationItem objects from each shard into a single vector](https://github.com/explosion/spacy-llm/blob/c87d5a6373485c7510d92b5fed08770f82c96c1a/spacy_llm/tasks/rel/util.py#L95-L100).

The comment in the code says this is fine, but RelationItem objects include the entity index of the dep/dest of the relationship. I think that's going to be 0-based for each shard, so the unpacked list will not have the right relationships for all shards after the first.