[fix] regression testing oss+sharded_ddp only (#281)

- tighter regression detection, based on the best case vs. worst case
- still run all configurations, useful for comparisons but not a target

.circleci/config.yml

@@ -135,7 +135,8 @@ run_oss_benchmark: &run_oss_benchmark
   - run:
       name: Run OSS Benchmark
       command: |
-        python benchmarks/oss.py --check_regression --world_size 4 --reference_speed 660 --reference_memory 1120 --reference_loss 0.023
+        python benchmarks/oss.py --world_size 4 --epochs 2
+        python benchmarks/oss.py --check_regression --world_size 4 --optim_type oss_sharded_ddp --reference_speed 660 --reference_memory 930 --reference_loss 0.023
 
 run_oss_gloo: &run_oss_gloo
   - run: