Update chain_of_thought.py (stanfordnlp#8006)

* Update chain_of_thought.py

Allow use of custom CoT representations (e.g., List of strings) instead of forcing CoT be a string output. 
Previously, even if the user passed in a custom definition for the reasoning field, the type of the reasoning field output would be a string. 
This change ensures that the type of the field is consistent with its annotation, and allows for users to specify a custom type for reasoning without creating a custom field for it. 
Also, this change introduces a docstring and type hints into the ChainOfThought module.

* Update chain_of_thought.py

Fixed error due to missing import.

* Update chain_of_thought.py

Avoid circular import and ensure there are two newlines between imports and initial code.

* Update chain_of_thought.py

Avoided circular import caused by referencing `dspy.OutputField`

Author: zbambergerNLP

dspy/predict/chain_of_thought.py

@@ -1,19 +1,36 @@
 import dspy
 from dspy.primitives.program import Module
-from dspy.signatures.signature import ensure_signature
+from dspy.signatures.field import OutputField
+from dspy.signatures.signature import ensure_signature, Signature
+from pydantic.fields import FieldInfo
+from typing import Optional, Union, Type
 
 
 class ChainOfThought(Module):
-    def __init__(self, signature, rationale_type=None, **config):
+    
+    def __init__(
+        self, 
+        signature: Type[Signature], 
+        rationale_field: Optional[Union[OutputField, FieldInfo]] = None, 
+        rationale_field_type: Type = str,
+        **config
+    ):
+        """
+        A module that reasons step by step in order to predict the output of a task.
+        
+        Args:
+            signature (Type[dspy.Signature]): The signature of the module.
+            rationale_field (Optional[Union[dspy.OutputField, pydantic.fields.FieldInfo]]): The field that will contain the reasoning.
+            rationale_field_type (Type): The type of the rationale field.
+            **config: The configuration for the module.
+        """
         super().__init__()
-
         signature = ensure_signature(signature)
-
         prefix = "Reasoning: Let's think step by step in order to"
         desc = "${reasoning}"
-        rationale_type = rationale_type or dspy.OutputField(prefix=prefix, desc=desc)
-        extended_signature = signature.prepend("reasoning", rationale_type, type_=str)
-        
+        rationale_field_type = rationale_field.annotation if rationale_field else rationale_field_type
+        rationale_field = rationale_field if rationale_field else dspy.OutputField(prefix=prefix, desc=desc)
+        extended_signature = signature.prepend(name="reasoning", field=rationale_field, type_=rationale_field_type)
         self.predict = dspy.Predict(extended_signature, **config)
 
     def forward(self, **kwargs):