[bug] There might be some memory leak

[nabenabe0928]

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Issue Description

From this PR, we have failures in unittest for cross validation in tabular classification.
If my guess is correct, this is due to some kind of memory leak.
After we reach the phase of experiments, we need to fix this issue so that we will not have serious memory leak.

[ravinkohli]

probably we should follow the recommendations and findings in #180

[nabenabe0928]

It is actually the one we caused in the last few PRs.
So we need to inspect the reason from preprocessors.