[RLlib] Structure and split up AlgorithmConfig.validate() method.

[sven1977]

Structure and split up AlgorithmConfig.validate() method.

• Single value checks should always happen directly in the individual setter methods, e.g. the correctness of a provided value for batch_mode should directly be checked in the rollouts() method and throw an error if not supported.
• The validate() method has been cleaned up/split into smaller, more manageable sub methods.

Why are these changes needed?

Related issue number

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