feat: add a basic async python client starting point (#1014)

This changes `lancedb` from a "pure python" setuptools project to a
maturin project and adds a rust lancedb dependency.

The async python client is extremely minimal (only `connect` and
`Connection.table_names` are supported). The purpose of this PR is to
get the infrastructure in place for building out the rest of the async
client.

Although this is not technically a breaking change (no APIs are
changing) it is still a considerable change in the way the wheels are
built because they now include the native shared library.

python/python/lancedb/rerankers/cohere.py

@@ -0,0 +1,81 @@
+import os
+from functools import cached_property
+from typing import Union
+
+import pyarrow as pa
+
+from ..util import attempt_import_or_raise
+from .base import Reranker
+
+
+class CohereReranker(Reranker):
+    """
+    Reranks the results using the Cohere Rerank API.
+    https://docs.cohere.com/docs/rerank-guide
+
+    Parameters
+    ----------
+    model_name : str, default "rerank-english-v2.0"
+        The name of the cross encoder model to use. Available cohere models are:
+        - rerank-english-v2.0
+        - rerank-multilingual-v2.0
+    column : str, default "text"
+        The name of the column to use as input to the cross encoder model.
+    top_n : str, default None
+        The number of results to return. If None, will return all results.
+    """
+
+    def __init__(
+        self,
+        model_name: str = "rerank-english-v2.0",
+        column: str = "text",
+        top_n: Union[int, None] = None,
+        return_score="relevance",
+        api_key: Union[str, None] = None,
+    ):
+        super().__init__(return_score)
+        self.model_name = model_name
+        self.column = column
+        self.top_n = top_n
+        self.api_key = api_key
+
+    @cached_property
+    def _client(self):
+        cohere = attempt_import_or_raise("cohere")
+        if os.environ.get("COHERE_API_KEY") is None and self.api_key is None:
+            raise ValueError(
+                "COHERE_API_KEY not set. Either set it in your environment or \
+                pass it as `api_key` argument to the CohereReranker."
+            )
+        return cohere.Client(os.environ.get("COHERE_API_KEY") or self.api_key)
+
+    def rerank_hybrid(
+        self,
+        query: str,
+        vector_results: pa.Table,
+        fts_results: pa.Table,
+    ):
+        combined_results = self.merge_results(vector_results, fts_results)
+        docs = combined_results[self.column].to_pylist()
+        results = self._client.rerank(
+            query=query,
+            documents=docs,
+            top_n=self.top_n,
+            model=self.model_name,
+        )  # returns list (text, idx, relevance) attributes sorted descending by score
+        indices, scores = list(
+            zip(*[(result.index, result.relevance_score) for result in results])
+        )  # tuples
+        combined_results = combined_results.take(list(indices))
+        # add the scores
+        combined_results = combined_results.append_column(
+            "_relevance_score", pa.array(scores, type=pa.float32())
+        )
+
+        if self.score == "relevance":
+            combined_results = combined_results.drop_columns(["score", "_distance"])
+        elif self.score == "all":
+            raise NotImplementedError(
+                "return_score='all' not implemented for cohere reranker"
+            )
+        return combined_results