Auto feature engineering (#1387)

* update readme in ga_squad

* update readme

* fix typo

* Update README.md

* Update README.md

* Update README.md

* update readme

* update

* fix path

* update reference

* fix bug in config file

* update nni_arch_overview.png

* update

* update

* update

* update home page

* update default value of metis tuner

* fix broken link in CommunitySharings

* update docs about nested search space

* update docs

* rename cascding to nested

* fix broken link

* update

* update issue link

* fix typo

* update evaluate parameters from GMM

* refine code

* fix optimized mode bug

* update import warning

* update warning

* update optimized mode

* add auto-feature-engineering

examples/trials/auto-feature-engineering/README.md

+ **Automatic Feature Engineering in nni**	
+ ===	
+ 
+  Now we have an [example](https://github.com/SpongebBob/tabular_automl_NNI), which could automaticlly do feature engineering in nni. 
+  
+  These code come from our contributors. And thanks our lovely contributors!
+  
+ And welcome more and more people to join us!