GPU nan_euclidean metric

[MarcYin]

Summary

Nan values within the tabular data is a common issue and simple data imputation methods, such as filling mean or zero, will cause bias in the searching results.

This is related to issue #3355, where the current implementation is not available for GPU. It would be good if this nan_euclidean metric exist for both CPU and GPU

Platform

Running on:

• [ ] CPU
• [x] GPU

Interface:

• [ ] C++
• [x] Python