[FEA] cuvs-bench needs to measure index conversion time

[cjnolet]

Currently, cuvs-bench does not measure the conversion time for appropriate indexes. This is ultimately causing users to get an unrealistic picture of the index build times, especially for Vamana where the conversion time can grow quite large. We need to report the conversion time.

cc @tarang-jain

[cjnolet]

@tarang-jain and @achirkin have pointed out that in order to implement this, we'll need to update the conversion so that it doesn't require serializing to disk each time. We will also need to update HNSW so that it doesn't need to serialize to disk each time.

Related: #1482

[achirkin]

Hi @cjnolet, actually we can measure the index conversion time as a part or regular index build using NVTX markers once #1529 is merged (see my comment to that PR)