Add benchmarks

[devonkinghorn]

It would be great to get some ANN performance benchmarks for SPTAG for single node and distributed. The repo doesn't say how well it scales.

My team wanted to use FAISS a few months ago but there was a problem with the License so we are now using HNSWlib even though it isn't quite as performant. We would love to know if this is a viable alternative to FAISS

https://github.com/erikbern/ann-benchmarks is a great repo for ANN benchmarks and it would be great if you could help the author add SPTAG benchmarks for single node and multiple node setups. The author may have some difficulties setting everything up.

If you can't help out in the above repo. It would still be useful to show your own performance benchmarks in the README.