fix checkpoint validation

[kurarrr]

What this patch does to fix the issue.

When checkpoint file is in GCS, an exception occurs and I fixed that.

Link to any relevant issues or pull requests.

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[tfujiwar]

LGTM. Thank you!!

[iizukak]

Ownership Approval

[tsawada]

Readability Approval

[iizukak]

/ready

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