Add Performance Tuning guide recipe (pytorch#1161)

* Performance Tuning Guide - initial commit

* Minor tweaks

* Switched profiling guide thumbnail to pytorch logo

* Converted Tuning Guide to 80 chars/line

* Split tuning guide into general, GPU-specific and distributed optimizations.

* WAR to fix generation of header for 1st section

* Minor fixes

* Implemented changes suggested during initial review

* Changed 'addition assignment' to 'addition'

* Removed sentences about 1 CPU core for DataParallel training

* Reordering of layers is recommended only for DDP(find_unused_parameters=True)

* Fixed formatting

* s/constructors/model constructors and s/match/roughly match

* Fixed typos

Author: szmigacz