HOTFIX Update JAX MNIST example's load_dataset function call

[sudhu2k]

Description

Below lines were throwing

train_ds = load_dataset("mnist", split="train", trust_remote_code=True)
    test_ds = load_dataset("mnist", split="test", trust_remote_code=True)

huggingface_hub.errors.HfHubHTTPError: Client error '404 Not Found' for url 'https://huggingface.co/api/datasets/mnist/revision/77f3279092a1c1579b2250db8eafed0ad422088c' (Request ID: Root=1-6977d10b-10f27f5e3cefd1b025b18524;cdc55ca3-14b0-4115-b9c6-8bcdb2fc52f1)
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404

Sorry, we can't find the page you are looking for.

This PR changes the path from "mnist" to "ylecun/mnist" to fix the path. The previous path likely got updated.
Similar error happened when downloading glue dataset.

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

This PR changes the path from "mnist" to "ylecun/mnist".

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[matthiasdiener]

Manually tested, appears to work fine.

[sudhu2k]

@wangye805 @wenchenvincent @ipanfilo, This fix seems to be working. Let us know if we can merge this. Thanks!

[wangye805]

LGTM