Add vision/mnist dataset

[laggui]

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

Closes #1157

Changes

At last, no more Python/HF dependency for the base MNISTDatset! 🙂

• Add explicit note for examples with python requirement for HF datasets
• Add vision dataset module (optional)
• Refactor MNISTDataset to download raw data from original source (with a nice little progress bar when a download happens)
• Update guide and examples to point to new dataset struct in vision

[codecov]

Codecov Report

Attention: 650 lines in your changes are missing coverage. Please review.

Comparison is base (4ca3e31) 85.96% compared to head (80ff22f) 84.47%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
burn-core/src/record/serde/de.rs	39.45%	287 Missing ⚠️
burn-dataset/src/vision/mnist.rs	0.00%	134 Missing ⚠️
burn-core/src/record/serde/ser.rs	46.66%	96 Missing ⚠️
burn-dataset/src/vision/downloader.rs	0.00%	34 Missing ⚠️
burn-import/pytorch-tests/tests/boolean/mod.rs	0.00%	27 Missing ⚠️
burn-core/src/record/serde/data.rs	79.83%	24 Missing ⚠️
burn-import/src/pytorch/recorder.rs	61.53%	15 Missing ⚠️
burn-core/src/record/serde/adapter.rs	70.00%	12 Missing ⚠️
burn-core/src/record/serde/error.rs	0.00%	10 Missing ⚠️
burn-import/src/pytorch/reader.rs	91.22%	5 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
- Coverage   85.96%   84.47%   -1.50%     
==========================================
  Files         522      546      +24     
  Lines       59179    61296    +2117     
==========================================
+ Hits        50876    51777     +901     
- Misses       8303     9519    +1216     

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[nathanielsimard]

LGTM. I think I would add a description of the new feature flag before merging. (burn/burn/src/lib.rs)

[laggui]

LGTM. I think I would add a description of the new feature flag before merging. (burn/burn/src/lib.rs)

Done! Didn't realize we had this description.

[antimora]

Looks good to me! Thank for the enhancement.