implement embedding generation in supported inference providers (#589)

This PR adds the ability to generate embeddings in all supported
inference providers.

```
pytest -v -s llama_stack/providers/tests/inference/test_embeddings.py -k "bedrock" --inference-model="amazon.titan-embed-text-v2:0"  --env EMBEDDING_DIMENSION=1024

 pytest -v -s -k "vllm"  --inferrence-model="intfloat/e5-mistral-7b-instruct"  llama_stack/providers/tests/inference/test_embeddings.py --env EMBEDDING_DIMENSION=4096  --env VLLM_URL="http://localhost:9798/v1"

pytest -v -s --inference-model="nomic-ai/nomic-embed-text-v1.5"  llama_stack/providers/tests/inference/test_embeddings.py  -k "fireworks"  --env FIREWORKS_API_KEY=<API_KEY>--env EMBEDDING_DIMENSION=128

pytest -v -s --inference-model="togethercomputer/m2-bert-80M-2k-retrieval"  llama_stack/providers/tests/inference/test_embeddings.py  -k "together"  --env TOGETHER_API_KEY=<API_KEY>--env EMBEDDING_DIMENSION=768

pytest -v -s -k "ollama"  --inference-model="all-minilm:v8"  llama_stack/providers/tests/inference/test_embeddings.py --env EMBEDDING_DIMENSION=384

 torchrun $CONDA_PREFIX/bin/pytest -v -s -k "meta_reference" --inference-model="sentence-transformers/all-MiniLM-L6-v2"  llama_stack/providers/tests/inference/test_embeddings.py --env EMBEDDING_DIMENSION=384

```

llama_stack/providers/utils/inference/embedding_mixin.py

@@ -0,0 +1,47 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import logging
+from typing import List
+
+from llama_models.llama3.api.datatypes import InterleavedTextMedia
+
+from llama_stack.apis.inference.inference import EmbeddingsResponse, ModelStore
+
+EMBEDDING_MODELS = {}
+
+
+log = logging.getLogger(__name__)
+
+
+class SentenceTransformerEmbeddingMixin:
+    model_store: ModelStore
+
+    async def embeddings(
+        self,
+        model_id: str,
+        contents: List[InterleavedTextMedia],
+    ) -> EmbeddingsResponse:
+        model = await self.model_store.get_model(model_id)
+        embedding_model = self._load_sentence_transformer_model(
+            model.provider_resource_id
+        )
+        embeddings = embedding_model.encode(contents)
+        return EmbeddingsResponse(embeddings=embeddings)
+
+    def _load_sentence_transformer_model(self, model: str) -> "SentenceTransformer":
+        global EMBEDDING_MODELS
+
+        loaded_model = EMBEDDING_MODELS.get(model)
+        if loaded_model is not None:
+            return loaded_model
+
+        log.info(f"Loading sentence transformer for {model}...")
+        from sentence_transformers import SentenceTransformer
+
+        loaded_model = SentenceTransformer(model)
+        EMBEDDING_MODELS[model] = loaded_model
+        return loaded_model