refresh LightningModule API docs (#8276)

* optimization docs

* example input array docs

* step and rank

* on_gpu

* auto-opt

* truncated backprop

* fixes

* manual backward

* backward

* all gather

* fix configure optimizers html

* whitespace

* toggle, untoggle

* scripting

* save hyperparameters

* optimizer step

* step functions

* hparams

* model size

* summarize

* header

* rm model summary

* Update pytorch_lightning/core/lightning.py

* Update pytorch_lightning/core/lightning.py

Author: awaelchli