[RLlib] on_environment_created callback called with incorrect arg names by EnvRunners.

[sven1977]

Bug fix:
RLlib's new EnvRunners (SingleAgentEnvRunner and MultiAgentEnvRunner) both call the on_environment_created callback with a wrong set of args: env_context should be used instead of env_config and env_index should not be used b/c on the new stack, the gym.env (or gym.vector.Env) is only created once and without a particular vector index for the individual sub-envs.

Why are these changes needed?

Related issue number

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[simonsays1980]

LGTM.