Update examples/custom_criterion.cu #343

With merge of #341 the stopping criterion with deterministic number of samples collected is available through CLI.

The example/custom_criterion.cu implemented exactly that, and it would be nice to update the example to implement some other stopping criterion.

Possible candidates:

• Minimum accumulated time reached
• Batched quantile stability (check every multiple of K samples, compare max relative change of 25-th, 50-th and 75-th percentiles and stop if below threshold.