Ensure performance meets internal benchmark

[Gamenot]

Benchmark,

1 agent, 50 SVs, loop scenario: 100 FPS
10 agents, 0 SVs, loop scenario, 25 FPS

Feel free to suggest a different benchmark than ^

Context:

davidrusu: Trying to run both agents and social vehicles together hides the performance hit from each individually.
Here's the curves of how increasing either value, slows our performance:

Tests were run on the loop scenario and Agents were instances of the following AgentSpec:

class Policy(AgentPolicy):
    def act(self, obs):
        return "keep_lane"

agent_spec = AgentSpec(
    interface=AgentInterface.from_type(AgentType.Laner, max_episode_steps=1000),
    policy_builder=Policy,
)

davidrusu: @jun Well, we can't directly control how many social vehicles are in the scenario, but here's a few episodes with a traffic flow of 30SV/min + 10 agents.

╭────────────────────┬────────────────────┬────────────────────┬────────────────────┬────────────────────┬╮
│            Episode │     Sim T / Wall T │        Total Steps │        Steps / Sec │       Scenario Map ││
├────────────────────┼────────────────────┼────────────────────┼────────────────────┼────────────────────┼┤
│              0/100 │               2.18 │                999 │              21.78 │               loop ││
│              1/100 │               1.32 │                999 │              13.23 │               loop ││
│              2/100 │               1.24 │                999 │              12.35 │               loop ││
│              3/100 │               1.59 │                999 │              15.87 │               loop ││

[wuyukun-tong]

运行vis.py的时候出现了这个错误是怎么回事，麻烦大神解答一下，谢谢
Successfully registered the following environments: ['MetaDrive-test-v0', 'MetaDrive-validation-v0', 'MetaDrive-v0', 'MetaDrive-10envs-v0', 'MetaDrive-1000envs-v0', 'MetaDrive-training0-v0', 'MetaDrive-training1-v0', 'MetaDrive-training2-v0'].
Traceback (most recent call last):
File "vis.py", line 83, in
env.render(mode="top_down", num_stack=25)
File "/home/gzz/miniconda3/envs/copo/lib/python3.7/site-packages/metadrive/envs/base_env.py", line 265, in render
return self._render_topdown(*args, **kwargs)
File "/home/gzz/miniconda3/envs/copo/lib/python3.7/site-packages/metadrive/envs/base_env.py", line 480, in _render_topdown
self._top_down_renderer = TopDownRenderer(*args, **kwargs)
File "/home/gzz/miniconda3/envs/copo/lib/python3.7/site-packages/metadrive/obs/top_down_renderer.py", line 143, in init
assert self.current_track_vehicle is not None, "Specify which vehicle to track"
File "/home/gzz/miniconda3/envs/copo/lib/python3.7/site-packages/metadrive/obs/top_down_renderer.py", line 235, in current_track_vehicle
return self.engine.agent_manager.active_agents["default_agent"]