Update input arguments of XGBoost to be compatible with the latest APIs

[jeongyoonlee]

Proposed changes

This PR fixes #787 by updating the input arguments of XGBoost to be compatible with the latest APIs. It moves eval_metric and early_stopping_rounds from fit() to __init__().

It also suppresses warnings from LightGBM in plot_tmle_gain().

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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)
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Checklist

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Further comments

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[paullo0106]

LGTM