Features
* heterogeneous cluster set up in distribution config
* support heterogeneous cluster for training
* include fields to work with inference recommender
Bug Fixes and Other Changes
* Moving the newly added field instance_group to the end of method
* image_uri does not need to be specified with instance_groups
* Loosen version of attrs dependency
* Add PipelineVariable annotation in estimatory, processing, tuner, transformer base classes
* model table link
Documentation Changes
* documentation for heterogeneous cluster