Add Mnist examples with lite

[tchaton]

What does this PR do?

This PR adds MNIST example written with pytorch, lite and Lightning.

Does your PR introduce any breaking changes? If yes, please list them.

Nope

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[awaelchli]

while the example itself is great, this adds much disorganization to the pl_examples folder as now the basic_examples folder has no meaning and the lite examples are in two different folders.

[rohitgr7]

small comments.. overall looks good 🚀