Update baseline test import and fix docstring (#170)

.circleci/config.yml

habitat_baselines/common/base_trainer.py

habitat_baselines/common/baseline_registry.py

habitat_baselines/common/env_utils.py

habitat_baselines/common/rollout_storage.py

habitat_baselines/common/tensorboard_utils.py

habitat_baselines/common/utils.py

habitat_baselines/rl/ppo/ppo_trainer.py

test/test_baseline_agents.py