Fix fine-tuned replicate models with faster cold boot (#10512)

With the latest support for faster cold boot in replicate
https://replicate.com/blog/fine-tune-cold-boots it looks like the
replicate LLM support in langchain is broken since some internal
replicate inputs are being returned.

Screenshot below illustrates the problem:

<img width="1917" alt="image"
src="https://github.com/langchain-ai/langchain/assets/749277/d28c27cc-40fb-4258-8710-844c00d3c2b0">

As you can see, the new replicate_weights param is being sent down with
x-order = 0 (which is causing langchain to use that param instead of
prompt which is x-order = 1)

FYI @baskaryan this requires a fix otherwise replicate is broken for
these models. I have pinged replicate whether they want to fix it on
their end by changing the x-order returned by them.

Update: per suggestion I updated the PR to just allow manually setting
the prompt_key which can be set to "prompt" in this case by callers... I
think this is going to be faster anyway than trying to dynamically query
the model every time if you know the prompt key for your model.

---------

Co-authored-by: Taqi Jaffri <tjaffri@docugami.com>

libs/langchain/langchain/llms/replicate.py

@@ -33,6 +33,7 @@ class Replicate(LLM):
     input: Dict[str, Any] = Field(default_factory=dict)
     model_kwargs: Dict[str, Any] = Field(default_factory=dict)
     replicate_api_token: Optional[str] = None
+    prompt_key: Optional[str] = None
 
     streaming: bool = Field(default=False)
     """Whether to stream the results."""
@@ -114,15 +115,18 @@ class Replicate(LLM):
         model = replicate_python.models.get(model_str)
         version = model.versions.get(version_str)
 
-        # sort through the openapi schema to get the name of the first input
-        input_properties = sorted(
-            version.openapi_schema["components"]["schemas"]["Input"][
-                "properties"
-            ].items(),
-            key=lambda item: item[1].get("x-order", 0),
-        )
-        first_input_name = input_properties[0][0]
-        inputs = {first_input_name: prompt, **self.input}
+        if not self.prompt_key:
+            # sort through the openapi schema to get the name of the first input
+            input_properties = sorted(
+                version.openapi_schema["components"]["schemas"]["Input"][
+                    "properties"
+                ].items(),
+                key=lambda item: item[1].get("x-order", 0),
+            )
+
+            self.prompt_key = input_properties[0][0]
+
+        inputs = {self.prompt_key: prompt, **self.input}
 
         prediction = replicate_python.predictions.create(
             version=version, input={**inputs, **kwargs}