Fix Xbox controller; introduce Takeover policy w/o brake

[pengzhenghao]

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[pengzhenghao]

@QuanyiLi could you play with drive_in_single_agent_env.py to see if this feels better?

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[QuanyiLi]

It feels bad when the car drives fast. At least, my performance degrades. Maybe I overfit the old keyboard controller...

[QuanyiLi]

It takes so long for the steering wheel to return to the center. It makes the control hard when the speed is high.

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[pengzhenghao]

Now I've reverted all values to previous version. The key differences:

1. we introduce space key to indicate takeover when using keyboard
2. A little different behavior on up and down keys:

If throttle > 0, and you press down, we will not set the brake to -brake_increment. The original code is:

        elif throttle_brake < 0.:   # WHEN YOU PRESS DOWN KEY
            self.throttle_brake = min(self.throttle_brake, 0.)  # IF throttle>0, IT WILL SET TO 0.
            self.throttle_brake -= self.BRAKE_INCREMENT  # throttle NOW EQUALS brake_increment

Instead, in this impl, we use:

        elif down_key_pressed:
            if self.throttle_brake <= 0.0:
                self.throttle_brake -= self.BRAKE_INCREMENT
            else:
                self.throttle_brake -= self.BRAKE_INCREMENT_WHEN_THROTTLE

note that

BRAKE_INCREMENT = 0.5
BRAKE_INCREMENT_WHEN_THROTTLE = 0.5

In this impl, we avoid too large change in the throttle_brake.

---

note: when I say press down, I say press s. Same for up.

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[QuanyiLi]

It is good now.

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[BOT] Great job! You have changed the docstring coverage from 30.54% to 30.55%, improving 0.012%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 30.54% to 30.55%, improving 0.012%.