Do a more precise check whether a dataset already exists

If the dataset folder exists, but is empty, then `read_from_directory` fails.

PiperOrigin-RevId: 649046864

Author: tomvdw

tensorflow_datasets/core/dataset_builder.py

@@ -286,7 +286,8 @@ def __init__(
     self._version = self._pick_version(version)
     # Compute the base directory (for download) and dataset/version directory.
     self._data_dir_root, self._data_dir = self._build_data_dir(data_dir)
-    if self.data_path.exists():
+    # If the dataset info is available, use it.
+    if dataset_info.dataset_info_path(self.data_path).exists():
       self.info.read_from_directory(self._data_dir)
     else:  # Use the code version (do not restore data)
       self.info.initialize_from_bucket()