Support S3 Object keys with `/` in `S3FileLoader` (#1517)

Resolves https://github.com/hwchase17/langchain/issues/1510

### Problem
When loading S3 Objects with `/` in the object key (eg.
`folder/some-document.txt`) using `S3FileLoader`, the objects are
downloaded into a temporary directory and saved as a file.

This errors out when the parent directory does not exist within the
temporary directory.

See
https://github.com/hwchase17/langchain/issues/1510#issuecomment-1459583696
on how to reproduce this bug

### What this pr does
Creates parent directories based on object key. 

This also works with deeply nested keys:
`folder/subfolder/some-document.txt`

langchain/document_loaders/s3_file.py

@@ -1,4 +1,5 @@
 """Loading logic for loading documents from an s3 file."""
+import os
 import tempfile
 from typing import List
 
@@ -27,6 +28,7 @@ class S3FileLoader(BaseLoader):
         s3 = boto3.client("s3")
         with tempfile.TemporaryDirectory() as temp_dir:
             file_path = f"{temp_dir}/{self.key}"
+            os.makedirs(os.path.dirname(file_path), exist_ok=True)
             s3.download_file(self.bucket, self.key, file_path)
             loader = UnstructuredFileLoader(file_path)
             return loader.load()