unstructured, community, initialize langchain-unstructured package (#22779)

#### Update (2): 
A single `UnstructuredLoader` is added to handle both local and api
partitioning. This loader also handles single or multiple documents.

#### Changes in `community`:
Changes here do not affect users. In the initial process of using the
SDK for the API Loaders, the Loaders in community were refactored.
Other changes include:
The `UnstructuredBaseLoader` has a new check to see if both
`mode="paged"` and `chunking_strategy="by_page"`. It also now has
`Element.element_id` added to the `Document.metadata`.
`UnstructuredAPIFileLoader` and `UnstructuredAPIFileIOLoader`. As such,
now both directly inherit from `UnstructuredBaseLoader` and initialize
their `file_path`/`file` attributes respectively and implement their own
`_post_process_elements` methods.

--------
#### Update:
New SDK Loaders in a [partner
package](https://python.langchain.com/v0.1/docs/contributing/integrations/#partner-package-in-langchain-repo)
are introduced to prevent breaking changes for users (see discussion
below).

##### TODO:
- [x] Test docstring examples
--------
- **Description:** UnstructuredAPIFileIOLoader and
UnstructuredAPIFileLoader calls to the unstructured api are now made
using the unstructured-client sdk.
- **New Dependencies:** unstructured-client

- [x] **Add tests and docs**: If you're adding a new integration, please
include
- [x] a test for the integration, preferably unit tests that do not rely
on network access,
- [x] update the description in
`docs/docs/integrations/providers/unstructured.mdx`
- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

TODO:
- [x] Update
https://python.langchain.com/v0.1/docs/integrations/document_loaders/unstructured_file/#unstructured-api
-
`langchain/docs/docs/integrations/document_loaders/unstructured_file.ipynb`
- The description here needs to indicate that users should install
`unstructured-client` instead of `unstructured`. Read over closely to
look for any other changes that need to be made.
- [x] Update the `lazy_load` method in `UnstructuredBaseLoader` to
handle json responses from the API instead of just lists of elements.
- This method may need to be overwritten by the API loaders instead of
changing it in the `UnstructuredBaseLoader`.
- [x] Update the documentation links in the class docstrings (the
Unstructured documents have moved)
- [x] Update Document.metadata to include `element_id` (see thread
[here](https://unstructuredw-kbe4326.slack.com/archives/C044N0YV08G/p1718187499818419))

---------

Signed-off-by: ChengZi <chen.zhang@zilliz.com>
Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: Isaac Francisco <78627776+isahers1@users.noreply.github.com>
Co-authored-by: ChengZi <chen.zhang@zilliz.com>

libs/partners/unstructured/README.md

@@ -0,0 +1,71 @@
+# langchain-unstructured
+
+This package contains the LangChain integration with Unstructured
+
+## Installation
+
+```bash
+pip install -U langchain-unstructured
+```
+
+And you should configure credentials by setting the following environment variables:
+
+```bash
+export UNSTRUCTURED_API_KEY="your-api-key"
+```
+
+## Loaders
+
+Partition and load files using either the `unstructured-client` sdk and the
+Unstructured API or locally using the `unstructured` library.
+
+API:
+To partition via the Unstructured API `pip install unstructured-client` and set
+`partition_via_api=True` and define `api_key`. If you are running the unstructured API
+locally, you can change the API rule by defining `url` when you initialize the
+loader. The hosted Unstructured API requires an API key. See the links below to
+learn more about our API offerings and get an API key.
+
+Local:
+By default the file loader uses the Unstructured `partition` function and will
+automatically detect the file type.
+
+In addition to document specific partition parameters, Unstructured has a rich set
+of "chunking" parameters for post-processing elements into more useful text segments
+for uses cases such as Retrieval Augmented Generation (RAG). You can pass additional
+Unstructured kwargs to the loader to configure different unstructured settings.
+
+Setup:
+```bash
+    pip install -U langchain-unstructured
+    pip install -U unstructured-client
+    export UNSTRUCTURED_API_KEY="your-api-key"
+```
+
+Instantiate:
+```python
+from langchain_unstructured import UnstructuredLoader
+
+loader = UnstructuredLoader(
+    file_path = ["example.pdf", "fake.pdf"],
+    api_key=UNSTRUCTURED_API_KEY,
+    partition_via_api=True,
+    chunking_strategy="by_title",
+    strategy="fast",
+)
+```
+
+Load:
+```python
+docs = loader.load()
+
+print(docs[0].page_content[:100])
+print(docs[0].metadata)
+```
+
+References
+----------
+https://docs.unstructured.io/api-reference/api-services/sdk
+https://docs.unstructured.io/api-reference/api-services/overview
+https://docs.unstructured.io/open-source/core-functionality/partitioning
+https://docs.unstructured.io/open-source/core-functionality/chunking