[RLlib] Fix access to self._minibatch_size

[ArturNiederfahrenhorst]

Description

In IMPALA, we access an attribute self._minibatch_size which does not exist anymore.
It should be self._minibatch_size. While this check is nice, it's effectively untested code.
This PR introduces a test that adds a small test that triggers the relevant code path.

[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where an incorrect attribute self._minibatch_size was accessed in the IMPALA algorithm's configuration validation. The correct attribute self.minibatch_size is now used. Additionally, a new test is introduced to cover this validation logic, which was previously untested. The fix is correct and the addition of the test is a great improvement. I've added one suggestion to enhance the new test by parameterizing it and adding another validation scenario to make it more comprehensive and maintainable.

[gemini-code-assist]

Great job adding a test for this validation logic! To make it even more robust and easier to maintain, I suggest parameterizing the test using pytest.mark.parametrize. This allows us to easily cover more scenarios. I've included a second test case to also validate the check against total_train_batch_size, which is part of the same validation logic.

    @pytest.mark.parametrize(
        "minibatch_size, rollout_fragment_length, match",
        [
            (100, 30, r"`minibatch_size` \(100\) must either be None or a multiple of `rollout_fragment_length` \(30\)"),
            (600, 30, r"smaller than or equal to `total_train_batch_size` \(500\)"),
        ],
    )
    def test_impala_minibatch_size_check(
        self, minibatch_size, rollout_fragment_length, match
    ):
        config = (
            impala.IMPALAConfig()
            .environment("CartPole-v1")
            .training(minibatch_size=minibatch_size)
            .env_runners(rollout_fragment_length=rollout_fragment_length)
        )

        with pytest.raises(ValueError, match=match):
            config.validate()