[FEA] Improve connectivity of CAGRA graph index for search

[anaruse]

It has been empirically shown that the graph indexes generated by CAGRA for search provide comparable search accuracy to other libraries, but reachability from any node to all nodes is not guaranteed. In fact, it has been confirmed that the number of strongly connected components (SCC) of graph indexed created by CAGRA is not 1 in some 100M scale datasets.

This problem can be alleviated by increasing the number of degrees in the search graph, but this would increase the size of the graph index. It is desirable to address this problem without increasing the number of degrees of the search graph.

Prior study has shown that this problem can be solved by using a Minimum Spanning Tree (MST)-like approach, but in general, MST takes a long time. However, what is needed here is not an exact MST, but, for example, an approximate MST in which the total number of edges is not necessarily minimum. Such an approximate MST could be computed quickly on GPUs.