[REVIEW] Add float64 warning when loading LightGBM model

[wphicks]

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[GPUtester]

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[wphicks]

Resolve #2817.

I spent a little bit of time trying to understand whether there were other places in the code we could intercept uses of load_using_treelite_handle or load_from_treelite_model and offer warnings, but I didn't see anywhere that really made sense to do so. Do we think it is worth adding warnings directly to those methods indicating that there may be an issue even if we can't reliably detect if there will be an issue?

Otherwise, this change is pretty darn simple. Anything more sophisticated will probably have to wait on the Treelite update.

[wphicks]

@hcho3 and/or @JohnZed: If you wouldn't mind reviewing this, it would be much appreciated.

[hcho3]

I think this is the best we can do without updating Treelite module in cuML.