Word2vec loader - Code changes and unit tests.

1. Refactored word2vec loader to perform existing file checks before downloading or extracting.

2. Added units tests to load, download and extract functions.

utils_nlp/pretrained_embeddings/word2vec.py

@@ -16,7 +16,6 @@ def _extract_word2vec_vectors(zip_path, dest_filepath):
     Args:
         zip_path: Path to the downloaded compressed file.
         dest_filepath: Final destination file path to the extracted zip file.
-
     """
 
     if os.path.exists(zip_path):