Fix subclassing OpenAIEmbeddings (#4500)
# Fix subclassing OpenAIEmbeddings
Fixes #4498
## Before submitting
- Problem: Due to annotated type `Tuple[()]`.
- Fix: Change the annotated type to "Iterable[str]". Even though
tiktoken use
[Collection[str]](https://github.com/openai/tiktoken/blob/095924e02c85617df6889698d94515f91666c7ea/tiktoken/core.py#L80)
type annotation, but pydantic doesn't support Collection type, and
[Iterable](https://docs.pydantic.dev/latest/usage/types/#typing-iterables)
is the closest to Collection.
