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/tests/inference/test_embeddings.py

@@ -0,0 +1,62 @@
+# 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 pytest
+
+from llama_stack.apis.inference import EmbeddingsResponse, ModelType
+
+# How to run this test:
+# pytest -v -s llama_stack/providers/tests/inference/test_embeddings.py
+
+
+class TestEmbeddings:
+    @pytest.mark.asyncio
+    async def test_embeddings(self, inference_model, inference_stack):
+        inference_impl, models_impl = inference_stack
+        model = await models_impl.get_model(inference_model)
+
+        if model.model_type != ModelType.embedding_model:
+            pytest.skip("This test is only applicable for embedding models")
+
+        response = await inference_impl.embeddings(
+            model_id=inference_model,
+            contents=["Hello, world!"],
+        )
+        assert isinstance(response, EmbeddingsResponse)
+        assert len(response.embeddings) > 0
+        assert all(isinstance(embedding, list) for embedding in response.embeddings)
+        assert all(
+            isinstance(value, float)
+            for embedding in response.embeddings
+            for value in embedding
+        )
+
+    @pytest.mark.asyncio
+    async def test_batch_embeddings(self, inference_model, inference_stack):
+        inference_impl, models_impl = inference_stack
+        model = await models_impl.get_model(inference_model)
+
+        if model.model_type != ModelType.embedding_model:
+            pytest.skip("This test is only applicable for embedding models")
+
+        texts = ["Hello, world!", "This is a test", "Testing embeddings"]
+
+        response = await inference_impl.embeddings(
+            model_id=inference_model,
+            contents=texts,
+        )
+
+        assert isinstance(response, EmbeddingsResponse)
+        assert len(response.embeddings) == len(texts)
+        assert all(isinstance(embedding, list) for embedding in response.embeddings)
+        assert all(
+            isinstance(value, float)
+            for embedding in response.embeddings
+            for value in embedding
+        )
+
+        embedding_dim = len(response.embeddings[0])
+        assert all(len(embedding) == embedding_dim for embedding in response.embeddings)