[RLlib] Provide more constants for common result dict keys, e.g. EPISODE_RETURN_MEAN.

[sven1977]

Provide more constants for common result dict keys, e.g. EPISODE_RETURN_MEAN.

• EPISODE_RETURN_MEAN
• EPISODE_RETURN_MAX
• EPISODE_RETURN_MIN

Switch all remaining "sampler_results" to ENV_RUNNER_RESULTS, "evaluation" to EVALUATION_RESULTS, etc.. in all yamls and py files.

Why are these changes needed?

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[simonsays1980]

LGTM.

[simonsays1980]

Nice!