Chroma: Catch and handle `NotEnoughElementsException` (#3368)
## Problem and Solution
This PR solves #1793, which is more of a convenience for users than
anything else. When using Chroma as a vectorstore, if you try to run
similarity search with a `k` value that is larger than the number of
documents stored in the vectorstore, Chroma will raise a
`chromadb.errors.NotEnoughElementsException`.
The workaround is to add a new parameter in all similarity search
methods under the `Chroma` class called `find_highest_possible_k`, an
optional boolean parameter that defaults to True (changes default
behavior). If this parameter is set to `False`, the methods will behave
exactly as they did before this PR.
If the parameter is `True`, however, the method will try running
similarity search with the given `k`, and if
`chromadb.errors.NotEnoughElementsException` is raised, iteratively
lower `k` (down to `k=1`) until the error is no longer raised.
The following is an example of how this is implemented in the
`Chroma.similarity_search` method.
https://github.com/preritdas/langchain/blob/e0846c2bcaafa4f54a193a6a7dfa8ed46480c326/langchain/vectorstores/chroma.py#L127-L159
We add the `find_highest_possible_k` parameter as `Optional` and
defaulting to True. We explain it briefly in the docstring. We wrap the
previous similarity search logic inside a private local function that
takes `k`. If `find_highest_possible_k` is False, we return that private
function, retaining previous behavior. If it is True, which it is by
default, we iteratively lower `k` (until it is 1) until we can find `k`
documents from the Chroma vectorstore.
## Example
You create a `Chroma` object from 1 document. You then run
`.similarity_search()`, `.similarity_search_by_vector()`, or
`similarity_search_with_score()`. If you only pass a query, the default
`k` is `4`. All methods would previously raise a
`chromadb.errors.NotEnoughElementsException`.
Now, however, all methods will return one document, the document inside
the vectorstore (unless you're filtering, setting a maximum distance,
etc.).
## Note
I didn't find any places in the documentation to mention this change,
other than the example Jupyter notebook for the Chroma vectorstore. In
that notebook, there was never a cell running similarity search with
parameters. If it's important to include information on altering the
`find_highest_possible_k` parameter, I'll happily document it wherever.
