add cohere encoder

decision_layer/encoders/cohere.py

+import os
+import cohere
 from decision_layer.encoders import BaseEncoder
 
 class CohereEncoder(BaseEncoder):
-    def __init__(self, name: str):
-        super().__init__(name)
+    client: cohere.Client | None
+    def __init__(self, name: str, cohere_api_key: str | None = None):
+        super().__init__(name=name, client=None)
+        cohere_api_key = cohere_api_key or os.getenv("COHERE_API_KEY")
+        if cohere_api_key is None:
+            raise ValueError("Cohere API key cannot be 'None'.")
+        self.client = cohere.Client(cohere_api_key)
 
     def __call__(self, texts: list[str]) -> list[float]:
-        raise NotImplementedError
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+        if len(texts) == 1:
+            input_type = "search_query"
+        else:
+            input_type = "search_document"
+        embeds = self.client.embed(
+            texts, input_type=input_type, model="embed-english-v3.0"
+        )
+        return embeds.embeddings
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