[rllib] IMPALA fix no attribute '_minibatch_size'

[lukaskiss222]

Why are these changes needed?

Fix no attribute '_minibatch_size'

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• Testing Strategy
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[Copilot]

Pull Request Overview

Fixes an AttributeError by replacing references to the nonexistent internal _minibatch_size attribute with the public minibatch_size property in Impala’s validation logic.

• Updated error message to use self.minibatch_size instead of self._minibatch_size.

[lukaskiss222]

Plus want to verify if this code is correct:
419-422 impala.py

            if self.minibatch_size is not None and not (
                (self.minibatch_size % self.rollout_fragment_length == 0)
                and self.minibatch_size <= self.total_train_batch_size
            ):

Should not it be: ?

            if self.minibatch_size is not None and not (
                (self.rollout_fragment_length  % self.minibatch_size == 0)
                and self.minibatch_size <= self.total_train_batch_size
            ):

Swapped self.rollout_fragment_length and self.minibatch_size in modulo operation.

[github-actions]

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[pseudo-rnd-thoughts]

Resolved by #58595