Add jupyter-notebook training + test example

[alexeyhorkin]

Wouldn't it be nice if I added a jupyter-notebook example how to train end test effdet models on a custom dataset?
Is it would be useful?

[alexeyhorkin]

@rwightman =)

[rwightman]

@alexeyhorkin there's this example https://gist.github.com/Chris-hughes10/73628b1d8d6fc7d359b3dcbbbb8869d7 ... my training scripts aren't very notebook friendly right now and would require modifications, but this lightning based example should work

[rayennecandido]

amazing repo