### Summary
Adds a new document loader for processing e-publications. Works with
`unstructured>=0.5.4`. You need to have
[`pandoc`](https://pandoc.org/installing.html) installed for this loader
to work.
### Testing
```python
from langchain.document_loaders import UnstructuredEPubLoader
loader = UnstructuredEPubLoader("winter-sports.epub", mode="elements")
data = loader.load()
data[0]
```
### Summary
Allows users to pass in `**unstructured_kwargs` to Unstructured document
loaders. Implemented with the `strategy` kwargs in mind, but will pass
in other kwargs like `include_page_breaks` as well. The two currently
supported strategies are `"hi_res"`, which is more accurate but takes
longer, and `"fast"`, which processes faster but with lower accuracy.
The `"hi_res"` strategy is the default. For PDFs, if `detectron2` is not
available and the user selects `"hi_res"`, the loader will fallback to
using the `"fast"` strategy.
### Testing
#### Make sure the `strategy` kwarg works
Run the following in iPython to verify that the `"fast"` strategy is
indeed faster.
```python
from langchain.document_loaders import UnstructuredFileLoader
loader = UnstructuredFileLoader("layout-parser-paper-fast.pdf", strategy="fast", mode="elements")
%timeit loader.load()
loader = UnstructuredFileLoader("layout-parser-paper-fast.pdf", mode="elements")
%timeit loader.load()
```
On my system I get:
```python
In [3]: from langchain.document_loaders import UnstructuredFileLoader
In [4]: loader = UnstructuredFileLoader("layout-parser-paper-fast.pdf", strategy="fast", mode="elements")
In [5]: %timeit loader.load()
247 ms ± 369 µs per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [6]: loader = UnstructuredFileLoader("layout-parser-paper-fast.pdf", mode="elements")
In [7]: %timeit loader.load()
2.45 s ± 31 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```
#### Make sure older versions of `unstructured` still work
Run `pip install unstructured==0.5.3` and then verify the following runs
without error:
```python
from langchain.document_loaders import UnstructuredFileLoader
loader = UnstructuredFileLoader("layout-parser-paper-fast.pdf", mode="elements")
loader.load()
```
### Summary
Updates the docs to remove the `nltk` download steps from
`unstructured`. As of `unstructured` `0.4.14`, this is handled
automatically in the relevant modules within `unstructured`.
