CAGRA graph optimizer: clamp rev_graph_count

[tfeher]

This PR fixes a potential out of bounds access.

While building the reverse graph, we count the number of potential edges in rev_graph_count

raft/cpp/include/raft/neighbors/detail/cagra/graph_core.cuh

Lines 204 to 205 in 047bfb2

	const uint32_t pos = atomicAdd(rev_graph_count + dest_id, 1);
	if (pos < degree) { rev_graph[pos + ((uint64_t)degree * dest_id)] = src_id; }

While we store only max output_graph_degree neighbors, the rev_graph_count can be larger than output_graph_degree. The loop that uses rev_graph is updated to take this limit into account.

[cjnolet]

/merge