Do not save checkpoint in benchmark mode

Created by: jieru-hu

Patch Description thanks @stephenroller for the pointer!

Skip saving the checkpoints in when benchmarking

Testing steps Describe how you tested your changes tested by running a tiny benchmark

python -m metaseq.launcher.opt_baselines --model-size 8m --benchmark -t 1 -g 8 -n 1 -p test-8m --azure

verified in train.log

2022-08-09 23:47:29 | INFO | metaseq.checkpoint_utils | Not saving checkpoints.

update in 23b68a99

changed to warning

2022-08-10 00:09:03 | WARNING | metaseq.checkpoint_utils | Not saving checkpoints.