fix timing to determine slice size in grid search

[ytetra]

Fix timing to determine slice size. With this fix, error due to the empty part of the slice doesn't occur.
(Fixes: #215)

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This change is 

[k8s-ci-robot]

Hi @ytetra. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[YujiOshima]

/ok-to-test

[YujiOshima]

/retest

[YujiOshima]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YujiOshima

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[YujiOshima]

/retest