remove duplicated callback field (stanfordnlp#8230)

Author: TomeHirata

dspy/clients/lm.py

@@ -70,7 +70,6 @@ def __init__(
         self.provider = provider or self.infer_provider()
         self.callbacks = callbacks or []
         self.history = []
-        self.callbacks = callbacks or []
         self.num_retries = num_retries
         self.finetuning_model = finetuning_model
         self.launch_kwargs = launch_kwargs or {}