[ML] Add rerank task type to internal service

[davidkyle]

Implements the rerank task using cross encoder models uploaded to Elasticsearch using Eland and the _ml/trained_models APIs. The cross encoder model should be configured for text_similarity.

This example installs the BAAI/bge-reranker-base model:

docker run -it --rm elastic/eland \
    eland_import_hub_model \
      --cloud-id $CLOUD_ID \
      -u elastic -p $CLOUD_PWD \
      --hub-model-id BAAI/bge-reranker-base \
      --task-type text_similarity 

The rerank endpoint is created in the inference API with:

PUT _inference/rerank/bge-rerank
{
  "service": "elasticsearch",
  "service_settings": {
    "model_id": "baai__bge-reranker-base",
    "num_allocations": 1,
    "num_threads": 1
  },
  "task_settings": {
    "return_documents": true
  }  
}

There is one task setting return_documents which defaults to true. Return documents controls whether or not the response contains the input text.

Then to rerank:

POST _inference/rerank/bge-rerank
{
  "query": "How many people live in Berlin?",
  "input": [
    "Berlin has a population of 3,520,031 registered inhabitants in an area of 891.82 square kilometers.",
    "New York City is famous for the Metropolitan Museum of Art.",
    "Paris is the capital and most populous city of France. With an official estimated population of 2,102,650 residents"
  ]
}

The response is ordered by relevance, index links back to the original document position

{
  "rerank": [
    {
      "index": 0,
      "relevance_score": 9.379423,
      "text": "Berlin has a population of 3,520,031 registered inhabitants in an area of 891.82 square kilometers."
    },
    {
      "index": 2,
      "relevance_score": 0.83300024,
      "text": "Paris is the capital and most populous city of France. With an official estimated population of 2,102,650 residents"
    },
    {
      "index": 1,
      "relevance_score": -6.987791,
      "text": "New York City is famous for the Metropolitan Museum of Art."
    }
  ]
}

With return_documents: false the response does not contain the input text

POST _inference/rerank/bge-rerank
{
  "query": "How many people live in Berlin?",
  "input": [
    "Berlin has a population of 3,520,031 registered inhabitants in an area of 891.82 square kilometers.",
    "New York City is famous for the Metropolitan Museum of Art.",
    "Paris is the capital and most populous city of France. With an official estimated population of 2,102,650 residents"
  ],
  "task_settings" : {
    "return_documents": false
  }
}


{
  "rerank": [
    {
      "index": 0,
      "relevance_score": 9.379423
    },
    {
      "index": 2,
      "relevance_score": 0.83300024
    },
    {
      "index": 1,
      "relevance_score": -6.987791
    }
  ]
}

[elasticsearchmachine]

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

[elasticsearchmachine]

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