Fix incorrect feature importance calculation logic

[paullo0106]

Proposed changes

This fixes #402 by correcting the feature importance calculation logic

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have added tests that prove my fix is effective or that my feature works
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Further comments

NA

[jeongyoonlee]

LGTM. Thanks!