[RLlib] Fix MultiCallbacks on_sample_end signature.

[sven1977]

Fix MultiCallbacks on_sample_end signature.

Why are these changes needed?

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[sven1977]

This was the bug: MultiAgentEnvRunner and SingleAgentEnvRunner call this without the worker arg (but then env_runner is provided), leading to a

TypeError: make_multi_callbacks.<locals>._MultiCallbacks.on_sample_end() missing 1 required keyword-only argument: 'worker'

[simonsays1980]

Great catch! LGTM.