optimizer clean up

[tchaton]

What does this PR do?

This PR is a first attempt at cleaning out optimization by introducing LightningOptimizer.

Here is how SGD will get printed.

LightningSGD (
Parameter Group 0
    dampening: 0
    lr: 0.1
    momentum: 0
    nesterov: False
    weight_decay: 0
)

Fixes #4663

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

• In the file pytorch_lightning/core/lightning.py:

Line 1174:121: E501 line too long (193 > 120 characters)

• In the file pytorch_lightning/core/optimizer.py:

Line 78:121: E501 line too long (133 > 120 characters)

Comment last updated at 2020-11-30 23:35:35 UTC

[SeanNaren]

Great work @tchaton. I know there is a lot of discussion if this breaks pytorch-esque lightning but I think this is was needed. The amount of bugs/questions that boil down to incorrect override/usage of optimizer_step that this PR solves are huge.

[Borda]

still reading...

[Borda]

it is a huge change so let's hope we did not miss something...