Flake8 + Bugfixes + Linter

[Skylion007]

Motivation and Context

• This will do static code analysis on Habitat-Lab just like it soon will on Habitat-Sim

How Has This Been Tested

• I flake8'd the whole repo and made precommit hooks

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• [] I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #459 into master will decrease coverage by 0.01%.
The diff coverage is 80.85%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   77.69%   77.67%   -0.02%     
==========================================
  Files         110      110              
  Lines        8549     8497      -52     
==========================================
- Hits         6642     6600      -42     
+ Misses       1907     1897      -10     

Impacted Files	Coverage Δ
examples/vln_benchmark.py	0.00% <0.00%> (ø)
habitat/core/benchmark.py	47.94% <0.00%> (ø)
habitat/core/embodied_task.py	90.62% <0.00%> (ø)
...abitat/sims/habitat_simulator/habitat_simulator.py	94.39% <ø> (-0.03%)	⬇️
habitat/sims/pyrobot/pyrobot.py	84.48% <ø> (-0.14%)	⬇️
habitat/utils/__init__.py	100.00% <ø> (ø)
habitat_baselines/agents/ppo_agents.py	77.46% <ø> (+0.75%)	⬆️
habitat_baselines/agents/slam_agents.py	0.00% <0.00%> (ø)
habitat_baselines/common/env_utils.py	87.50% <ø> (-0.31%)	⬇️
habitat_baselines/rl/ddppo/policy/resnet.py	68.64% <ø> (-0.27%)	⬇️
... and 36 more

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