Replace `pl_examples` with tutorials.

[carmocca]

🚀 Feature

Motivation

https://github.com/PyTorchLightning/pytorch-lightning/tree/master/pl_examples has become a place for more tests which should be in tests instead. It defeats the purpose of being simple examples for users to read.

Pitch

Create tutorials for:
https://github.com/PyTorchLightning/pytorch-lightning/tree/master/pl_examples

After that, we can remove the pl_examples directory entirely and run the tests in the tests directory.