Add MNIST-C, a tutorial, & small fixes

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.05% :warning:

Comparison is base (5bd2490) 92.20% compared to head (4462356) 92.15%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ========================================== - Coverage 92.20% 92.15% -0.05% ========================================== Files 107 107 Lines 4718 4716 -2 Branches 439 441 +2 ========================================== - Hits 4350 4346 -4 + Misses 238 237 -1 - Partials 130 133 +3 ``` | Flag | Coverage Δ | | |---|---|---| | cpu | `92.15% <50.00%> (-0.05%)` | :arrow_down: | | pytest | `92.15% <50.00%> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ENSTA-U2IS#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/ENSTA-U2IS/torch-uncertainty/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ENSTA-U2IS) | Coverage Δ | | |---|---|---| | [...\_uncertainty/post\_processing/temperature\_scaler.py](https://app.codecov.io/gh/ENSTA-U2IS/torch-uncertainty/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ENSTA-U2IS#diff-dG9yY2hfdW5jZXJ0YWludHkvcG9zdF9wcm9jZXNzaW5nL3RlbXBlcmF0dXJlX3NjYWxlci5weQ==) | `100.00% <ø> (ø)` | | | [torch\_uncertainty/datamodules/mnist.py](https://app.codecov.io/gh/ENSTA-U2IS/torch-uncertainty/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ENSTA-U2IS#diff-dG9yY2hfdW5jZXJ0YWludHkvZGF0YW1vZHVsZXMvbW5pc3QucHk=) | `81.69% <50.00%> (-6.37%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/ENSTA-U2IS/torch-uncertainty/pull/38/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ENSTA-U2IS)

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