Fix MNIST datasets img path column

[shawnccx]

Use downloaded path instead of relative train/test path.

Code Pull Requests

Current MNIST example fails due to reading wrong paths of images.

[Errno 2] No such file or directory: '/Users/chongxiaoc/git/ludwig/examples/mnist/training/0/16585.png'
/Users/chongxiaoc/git/ludwig/ludwig/utils/image_utils.py:138: UserWarning: Failed to read image from PNG file. Original exception: Bytes object is empty. This could be due to a failed load from storage.
  warnings.warn(f"Failed to read image from PNG file. Original exception: {e}")
/Users/chongxiaoc/git/ludwig/ludwig/utils/image_utils.py:149: UserWarning: Failed to read image from numpy file. Original exception: Cannot load file containing pickled data when allow_pickle=False
  warnings.warn(f"Failed to read image from numpy file. Original exception: {e}")
/Users/chongxiaoc/git/ludwig/ludwig/utils/image_utils.py:120: UserWarning: Unable to read image from bytes object.
  warnings.warn("Unable to read image from bytes object.")

This PR fixes the img_path column in the dataframe.

[arnavgarg1]

@shawnccx Nice catch! Thanks for your contribution! Approved!

[github-actions]

Unit Test Results

       6 files  ±       0         6 suites  ±0   2h 51m 14s ⏱️ - 51m 15s
1 600 tests +1 446  1 566 ✔️ +1 430  33 💤 +15  1 ❌ +1 
1 633 runs  +1 305  1 593 ✔️ +1 307  39 💤  -   3  1 ❌ +1 

For more details on these failures, see this check.

Results for commit d1216dc. ± Comparison against base commit 662413e.

This pull request skips 5 tests.

tests.integration_tests.test_automl ‑ test_train_with_config_remote[s3]
tests.integration_tests.test_hyperopt ‑ test_hyperopt_sync_remote[s3]
tests.integration_tests.test_remote ‑ test_remote_training_set[s3-ray]
tests.regression_tests.benchmark.test_model_performance ‑ test_performance[ames_housing.gbm.yaml]
tests.regression_tests.benchmark.test_model_performance ‑ test_performance[mercedes_benz_greener.gbm.yaml]

[shawnccx]

@arnavgarg1 is it clean to merger?
The failed one test_model_save_reload_tv_model[googlenet-base] (tests.integration_tests.test_model_save_and_load) failed seems caused by http error and is not related with changes here.

[shawnccx]

@arnavgarg1 is it clean to merger? The failed one test_model_save_reload_tv_model[googlenet-base] (tests.integration_tests.test_model_save_and_load) failed seems caused by http error and is not related with changes here.

nvm, just saw it merged.