[Python]Embeddings API refactor (#580)

Sets things up for this -> https://github.com/lancedb/lancedb/issues/579
- Just separates out the registry/ingestion code from the function
implementation code
- adds a `get_registry` util
- package name "open-clip" -> "open-clip-torch"

python/tests/test_embeddings_slow.py

@@ -19,7 +19,7 @@ import pytest
 import requests
 
 import lancedb
-from lancedb.embeddings import EmbeddingFunctionRegistry
+from lancedb.embeddings import get_registry
 from lancedb.pydantic import LanceModel, Vector
 
 # These are integration tests for embedding functions.
@@ -31,7 +31,7 @@ from lancedb.pydantic import LanceModel, Vector
 @pytest.mark.parametrize("alias", ["sentence-transformers", "openai"])
 def test_sentence_transformer(alias, tmp_path):
     db = lancedb.connect(tmp_path)
-    registry = EmbeddingFunctionRegistry.get_instance()
+    registry = get_registry()
     func = registry.get(alias).create()
 
     class Words(LanceModel):
@@ -69,7 +69,7 @@ def test_openclip(tmp_path):
     from PIL import Image
 
     db = lancedb.connect(tmp_path)
-    registry = EmbeddingFunctionRegistry.get_instance()
+    registry = get_registry()
     func = registry.get("open-clip").create()
 
     class Images(LanceModel):
@@ -131,11 +131,7 @@ def test_openclip(tmp_path):
     os.environ.get("COHERE_API_KEY") is None, reason="COHERE_API_KEY not set"
 )  # also skip if cohere not installed
 def test_cohere_embedding_function():
-    cohere = (
-        EmbeddingFunctionRegistry.get_instance()
-        .get("cohere")
-        .create(name="embed-multilingual-v2.0")
-    )
+    cohere = get_registry().get("cohere").create(name="embed-multilingual-v2.0")
 
     class TextModel(LanceModel):
         text: str = cohere.SourceField()