Test MLPerf inference loadgen to automatically determine the thoughput of a system

[arjunsuresh]

We need to add in CM a way to automatically determine the throughput of a system using loadgen. This PR adds a <enforce_max_duration> option to loadgen - but not exactly sure, if it'll serve our purpose.

Currently, we have to provide an expected target_qps for a system and if we don't loadgen assumes this to be 1. What we ideally want is to keep loadgen running for say 5 minutes and then take the number of samples processed. So, the loadgen should keep sending the queries until 5 minutes are over.