Specify method for with_structured_output

[pbrady]

The default option of "function_calling" for Ollama was leading to errors. Defaulting to "json_schema" (as ChatOpenAI already does) results in success for Ollama. Expose the method argument to the user for finer-grained control if needed.

Fixes #38.

I'm not sure if this is the right approach but figured it would be a good starting point for a discussion. The script below used to trigger the parsing error mentioned in the issue. With the fix here, Ollama is able to produce Nodes and Relationships:

from langchain_ollama import ChatOllama
from langchain_experimental.graph_transformers import LLMGraphTransformer
from langchain_core.documents import Document

llm = ChatOllama(model="llama3.1", num_ctx=4096)
doc = Document(
    "The lazy dog watched as the red ball rolled.  Bob the clown scared the missles."
)

allowed_nodes = ["Animal", "Person", "Object"]

doc_transformer = LLMGraphTransformer(
    llm=llm, allowed_nodes=allowed_nodes
)

graph_doc = doc_transformer.convert_to_graph_documents([doc])
print(graph_doc)

Output:

GraphDocument(nodes=[Node(id='Lazy Dog', type='Person', properties={}), Node(id='Red Ball', type='Object', properties={}), Node(id='Bob The Clown', type='Person', properties={})], relationships=[Relationship(source=Node(id='Lazy Dog', type='Person', properties={}), target=Node(id='Red Ball', type='Object', properties={}), type='WATCHED', properties={}), Relationship(source=Node(id='Bob The Clown', type='Person', properties={}), target=Node(id='Missles', type='Object', properties={}), type='SCARED', properties={})], source=Document(metadata={}, page_content='The lazy dog watched as the red ball rolled.  Bob the clown scared the missles.'))]

[efriis]

(ignore initial review - thought this was modifying ChatOllama not this)

[efriis]

Sorry I misunderstood - let's pass this in as a kwarg when it's defined, but let's remove the default of json_schema above.

Also open to something more general like structured_output_kwargs: Optional[dict] where you'd achieve this with structured_output_kwargs={"method": "json_schema"}