Merge #4272

4272: Don't pass default revision when the model is explicitly set in config r=Kerollmops a=dureuill

# Pull Request

## Related issue
Fixes #4271 

## What does this PR do?

- When the `model` is explicitly set in the `embedders` setting, we reset the `revision` to `None`, such that if the user doesn't specify a revision, the head of the model repository is chosen. 
- Not changed: If the user specifies a revision, it applies, like previously. 
- Not changed: If the user doesn't specify a model, the default model with the default revision applies, like previously.

## Manual testing on a fresh DB

1. Enable experimental feature:
```sh
curl \
  -X PATCH 'http://localhost:7700/experimental-features/' \
  -H 'Content-Type: application/json' -H 'Authorization: Bearer foo' \
--data-binary '{ "vectorStore": true
  }'
```
2. Send settings with a specified model but no specified revision:
```sh
curl \
-X PATCH 'http://localhost:7700/indexes/products/settings' \
-H 'Content-Type: application/json' --data-binary \
'{ "embedders": { "default": { "source": { "huggingFace": { "model": "sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2" } }, "documentTemplate": { "template": "A product titled '{{doc.title}}'"} } } }'
```
3. Check that the task was successful:
```sh
curl 'http://localhost:7700/tasks/0'

{"uid":0,"indexUid":"products","status":"succeeded","type":"settingsUpdate","canceledBy":null,"details":{"embedders":{"default":{"source":{"huggingFace":{"model":"sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2"}},"documentTemplate":{"template":"A product titled {{doc.title}}"}}}},"error":null,"duration":"PT0.001892S","enqueuedAt":"2023-12-20T09:17:01.73789Z","startedAt":"2023-12-20T09:17:01.73854Z","finishedAt":"2023-12-20T09:17:01.740432Z"}
```
4. Send documents to index:
```sh
curl 'https://localhost:7700/indexes/products/documents' -H 'Content-Type: application/json' --data-binary '{"id": 0, "title": "Best product"}'
```

Co-authored-by: Louis Dureuil <louis@meilisearch.com>

milli/src/vector/settings.rs

@@ -228,6 +228,12 @@ impl From<HfEmbedderSettings> for crate::vector::hf::EmbedderOptions {
         let mut this = Self::default();
         if let Some(model) = model.set() {
             this.model = model;
+            // Reset the revision if we are setting the model.
+            // This allows the following:
+            // "huggingFace": {} -> default model with default revision
+            // "huggingFace": { "model": "name-of-the-default-model" } -> default model without a revision
+            // "huggingFace": { "model": "some-other-model" } -> most importantly, other model without a revision
+            this.revision = None;
         }
         if let Some(revision) = revision.set() {
             this.revision = Some(revision);