Fix mnist source

[mhabedank]

Code Pull Requests

The MNIST dataset could not get acquired from the original source because the data is not available anymore. This PR will solve the following issue #4009

Beside new tests the follow two lines will work again:

from ludwig.datasets import mnist

a, b = mnist.load(split=True)

[github-actions[bot]]

Unit Test Results

  6 files  ±0    6 suites  ±0   13m 52s :stopwatch: - 1m 7s 12 tests ±0    7 :heavy_check_mark:  -   2    5 :zzz: +  2  0 :x: ±0  60 runs  ±0  30 :heavy_check_mark:  - 12  30 :zzz: +12  0 :x: ±0 

Results for commit 07bd824e. ± Comparison against base commit 3b9192b7.

This pull request skips 2 tests.

``` 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] ```

:recycle: This comment has been updated with latest results.