Fix to_numpy_dataset() for Dask series

[hungcs]

    model.evaluate(
  File "/Users/hw/mambaforge/envs/based/lib/python3.8/site-packages/ludwig/api.py", line 914, in evaluate
    postproc_predictions = postprocess(
  File "/Users/hw/mambaforge/envs/based/lib/python3.8/site-packages/ludwig/data/postprocessing.py", line 43, in postprocess
    _save_as_numpy(predictions, output_directory, saved_keys)
  File "/Users/hw/mambaforge/envs/based/lib/python3.8/site-packages/ludwig/data/postprocessing.py", line 63, in _save_as_numpy
    numpy_predictions = to_numpy_dataset(predictions)
  File "/Users/hw/mambaforge/envs/based/lib/python3.8/site-packages/ludwig/utils/data_utils.py", line 386, in to_numpy_dataset
    dataset[col] = np.stack(df[col].to_numpy())
AttributeError: 'Series' object has no attribute 'to_numpy' (type: RayTaskError(AttributeError), retryable: true)

[github-actions]

Unit Test Results

       6 files  ±0         6 suites  ±0   2h 14m 7s ⏱️ - 19m 46s
2 802 tests +1  2 770 ✔️ +1    32 💤 ±0  0 ❌ ±0 
8 406 runs  +3  8 306 ✔️ +3  100 💤 ±0  0 ❌ ±0 

Results for commit 940e412. ± Comparison against base commit e08d765.

♻️ This comment has been updated with latest results.

[justinxzhao]

@hungcs Do you have a reference to an issue for where this came up? Could you also add a unit test to test_data_utils.py?

[hungcs]

@hungcs Do you have a reference to an issue for where this came up? Could you also add a unit test to test_data_utils.py?

@justinxzhao this happens when running model.evaluate() on the predibase side on the titanic dataset, as the dataset is a dask dataframe