Benchmark Example issue

The following command seems to fail. I use `torch=1.8.0+cu111`.
```
bfrun -np 4 python /home/hanbinhu/bluefog/test/../examples/pytorch_benchmark.py --model=lenet --num-iters=1 --dist-optimizer=gradient_allreduce
```
```
Traceback (most recent call last):
File "/home/hanbinhu/bluefog/test/../examples/pytorch_benchmark.py", line 138, in
bf.broadcast_optimizer_state(optimizer, root_rank=0)
File "/home/hanbinhu/miniconda3/envs/bluefog/lib/python3.8/site-packages/bluefog/torch/utility.py", line 201, in broadcast_optimizer_state
p = torch.Tensor([p])
TypeError: must be real number, not NoneType
```