Add lazy iteration interface to document loaders (#3659)
Adding a lazy iteration for document loaders.
Following the plan here:
https://github.com/hwchase17/langchain/pull/2833
Keeping the `load` method as is for backwards compatibility. The `load`
returns a materialized list of documents and downstream users may rely on that
fact.
A new method that returns an iterable is introduced for handling lazy
loading.
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Co-authored-by: Zander Chase <130414180+vowelparrot@users.noreply.github.com>
