Update `langchain.chains.create_extraction_chain_pydantic` to parse results successfully (#6887)

<!-- Thank you for contributing to LangChain!

Replace this comment with:
  - Description: a description of the change, 
  - Issue: the issue # it fixes (if applicable),
  - Dependencies: any dependencies required for this change,
- Tag maintainer: for a quicker response, tag the relevant maintainer
(see below),
- Twitter handle: we announce bigger features on Twitter. If your PR
gets announced and you'd like a mention, we'll gladly shout you out!

If you're adding a new integration, please include:
1. a test for the integration, preferably unit tests that do not rely on
network access,
  2. an example notebook showing its use.

Maintainer responsibilities:
  - General / Misc / if you don't know who to tag: @dev2049
  - DataLoaders / VectorStores / Retrievers: @rlancemartin, @eyurtsev
  - Models / Prompts: @hwchase17, @dev2049
  - Memory: @hwchase17
  - Agents / Tools / Toolkits: @vowelparrot
  - Tracing / Callbacks: @agola11
  - Async: @agola11

If no one reviews your PR within a few days, feel free to @-mention the
same people again.

See contribution guidelines for more information on how to write/run
tests, lint, etc:
https://github.com/hwchase17/langchain/blob/master/.github/CONTRIBUTING.md
 -->
 
- Description: 
- The current code uses `PydanticSchema.schema()` and
`_get_extraction_function` at the same time. As a result, a response
from OpenAI has two nested `info`, and
`PydanticAttrOutputFunctionsParser` fails to parse it. This PR will use
the pydantic class given as an arg instead.
- Issue: no related issue yet
- Dependencies: no dependency change
- Tag maintainer: @dev2049
- Twitter handle: @shotarok28

langchain/chains/openai_functions/extraction.py

@@ -78,9 +78,9 @@ def create_extraction_chain_pydantic(
     class PydanticSchema(BaseModel):
         info: List[pydantic_schema]  # type: ignore
 
-    openai_schema = PydanticSchema.schema()
+    openai_schema = pydantic_schema.schema()
     openai_schema = _resolve_schema_references(
-        openai_schema, openai_schema["definitions"]
+        openai_schema, openai_schema.get("definitions", {})
     )
 
     function = _get_extraction_function(openai_schema)