This PR is a documentation fix.
Description:
* fixes imports in the code samples in the docstrings of
`create_openai_fn_chain` and `create_structured_output_chain`
* fixes imports in
`docs/extras/modules/chains/how_to/openai_functions.ipynb`
* removes unused imports from the notebook
Issues:
* the docstrings use `from pydantic_v1 import BaseModel, Field` which
this PR changes to `from langchain.pydantic_v1 import BaseModel, Field`
* importing `pydantic` instead of `langchain.pydantic_v1` leads to
errors later in the notebook
