TypeError: add(): argument 'alpha' must be Number, not NoneType #35

I was trying to run the repo [DynamicHead](https://github.com/microsoft/DynamicHead) and trying to run the model with a custom dataset and have the following error. In the repo the launch function is called and I get these errors.

The command I run was
```
DETECTRON2_DATASETS=$DATASET python train_net.py --config configs/dyhead_r50_retina_fpn_1x.yaml --num-gpus 8
```
## Logs
```
Traceback (most recent call last):
File "train_net_custom.py", line 222, in
launch(
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/detectron2/engine/launch.py", line 82, in launch
main_func(*args)
File "train_net_custom.py", line 216, in main
return trainer.train()
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/detectron2/engine/defaults.py", line 484, in train
super().train(self.start_iter, self.max_iter)
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/detectron2/engine/train_loop.py", line 149, in train
self.run_step()
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/detectron2/engine/defaults.py", line 494, in run_step
self._trainer.run_step()
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/detectron2/engine/train_loop.py", line 294, in run_step
self.optimizer.step()
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/torch/optim/lr_scheduler.py", line 65, in wrapper
return wrapped(*args, **kwargs)
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/torch/optim/optimizer.py", line 89, in wrapper
return func(*args, **kwargs)
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/torch/autograd/grad_mode.py", line 27, in decorate_context
return func(*args, **kwargs)
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/torch/optim/sgd.py", line 110, in step
F.sgd(params_with_grad,
File "/home/xxx/anaconda3/envs/detectron/lib/python3.8/site-packages/torch/optim/_functional.py", line 160, in sgd
d_p = d_p.add(param, alpha=weight_decay)
TypeError: add(): argument 'alpha' must be Number, not NoneType
```
Detectron2 version: 0.6

## Expected behaviour is the training loop runs without error.