Refactor baselines: add generic trainer and common utils (#153)

* refactor and  add generic trainer class

* fix to pass tests

* change BaseModel to BaseTrainer

* fix tensorboard import causing CI failure

* modify circle-ci test script accordingly

* doc, typing and other changes

* rename BASELINE to TRAINER

* merge from upstream master

* Update Habitat-API to allow for no rendering sensors (#139)

Update Habitat-API to allow for no rendering sensors

* Added installation requirements step for sim installation in CI setup. (#159)

Added installation requirements step for sim installation in CI setup

* move RolloutStorage to utils

* add environments.py

* make ckpt dir if not exist

* small fixes according to comments

* changes according to comments

* update readme

* fix old config compatibility

* fix missed isort lint