measure_elapsed
elapsed = measure_elapsed(t, run_fn, run_ctx, CALIBRATE_SEED_BATCH, CALIBRATE_RUNS);
time_n = measure_elapsed(t, run_fn, run_ctx, t->batch_iters, CALIBRATE_RUNS);
time_2n = measure_elapsed(t, run_fn, run_ctx, t->batch_iters * 2, CALIBRATE_RUNS);