Minor visualizer_rllib.py bug

[kjang96]

The visualizer relies on the variable agent for computing actions.

The agent variable was mistakenly overwritten in for loops in between rollouts. I changed the for loop agent variable to be agent_id to resolve this issue.

[coveralls]

Pull Request Test Coverage Report for Build 1705

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 84.529%

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Files with Coverage Reduction	New Missed Lines	%
flow/envs/bay_bridge/base.py	2	64.96%

Totals
Change from base Build 1698:	0.0%
Covered Lines:	6682
Relevant Lines:	7905

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💛 - Coveralls

[eugenevinitsky]

Good catch!