a bug about test_net.py

After I updated the code version, this bug appeared in the test.I compared the code of the previous version and found that the problem really appeared in the updated code. I hope the official can fix it as soon as possible.
```
Traceback (most recent call last):
File "tools/run_net.py", line 52, in
main()
File "tools/run_net.py", line 37, in main
launch_job(cfg=cfg, init_method=args.init_method, func=test)
File "/opt/data/private/SlowFast/slowfast/utils/misc.py", line 430, in launch_job
func(cfg=cfg)
File "/opt/data/private/SlowFast/tools/test_net.py", line 252, in test
test_meter = perform_test(test_loader, model, test_meter, cfg, writer)
File "/root/miniconda3/envs/slowfast/lib/python3.8/site-packages/torch/autograd/grad_mode.py", line 27, in decorate_context
return func(*args, **kwargs)
File "/opt/data/private/SlowFast/tools/test_net.py", line 129, in perform_test
test_meter.iter_toc()
File "/opt/data/private/SlowFast/slowfast/utils/meters.py", line 159, in iter_toc
self.iter_timer.pause()
File "/root/miniconda3/envs/slowfast/lib/python3.8/site-packages/fvcore/common/timer.py", line 30, in pause
raise ValueError("Trying to pause a Timer that is already paused!")
ValueError: Trying to pause a Timer that is already paused!
```