fix: nltk data download path to prevent redundant nested directories (#3546)

Closes #3543.

### Summary
This PR addresses an issue with the NLTK data download process.
Previously, when downloading NLTK data, a nested "nltk_data" directory
was created within the parent "nltk_data" directory if the parent
directory already existed. This redundant directory structure led to two
significant problems:
- errors in checking if data had already been downloaded, potentially
causing redundant downloads in subsequent calls.
- failures in loading models from the downloaded NLTK data due to
incorrect path resolution.

This fix modifies the NLTK data download logic to prevent creation of
unnecessary nested directories. If the download path ends with
"nltk_data" and that directory already exists, we now use the existing
directory instead of creating a new nested one.

### Testing
CI should pass.

CHANGELOG.md

@@ -1,3 +1,13 @@
+## 0.15.7
+
+### Enhancements
+
+### Features
+
+### Fixes
+
+* **Fix NLTK data download path to prevent nested directories**. Resolved an issue where a nested "nltk_data" directory was created within the parent "nltk_data" directory when it already existed. This fix prevents errors in checking for existing downloads and loading models from NLTK data.
+
 ## 0.15.6
 
 ### Enhancements
@@ -10,7 +20,6 @@
 * **Update CI for `ingest-test-fixture-update-pr` to resolve NLTK model download errors.**
 * **Synchronized text and html on `TableChunk` splits.** When a `Table` element is divided during chunking to fit the chunking window, `TableChunk.text` corresponds exactly with the table text in `TableChunk.metadata.text_as_html`, `.text_as_html` is always parseable HTML, and the table is split on even row boundaries whenever possible.
 
-
 ## 0.15.5
 
 ### Enhancements

unstructured/__version__.py

@@ -1 +1 @@
-__version__ = "0.15.6"  # pragma: no cover
+__version__ = "0.15.7"  # pragma: no cover

unstructured/nlp/tokenize.py

@@ -70,6 +70,10 @@ def download_nltk_packages():
     if nltk_data_dir is None:
         raise OSError("NLTK data directory does not exist or is not writable.")
 
+    # Check if the path ends with "nltk_data" and remove it if it does
+    if nltk_data_dir.endswith("nltk_data"):
+        nltk_data_dir = os.path.dirname(nltk_data_dir)
+
     def sha256_checksum(filename: str, block_size: int = 65536):
         sha256 = hashlib.sha256()
         with open(filename, "rb") as f: