community: add len() implementation to Chroma (#19419)
Thank you for contributing to LangChain!
- [x] **Add len() implementation to Chroma**: "package: community"
- [x] **PR message**:
- **Description:** add an implementation of the __len__() method for the
Chroma vectostore, for convenience.
- **Issue:** no exposed method to know the size of a Chroma vectorstore
- **Dependencies:** None
- **Twitter handle:** lowrank_adrian
- [x] **Add tests and docs**
- [x] **Lint and test**
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Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>
