## Bug
When I repeatedly invoke build_detection_model function to build the model, I will get the error: `AttributeError: 'ResNet' object has no attribute 'layer1'`. 

## To Reproduce
```
model = build_detection_model(cfg)
model = build_detection_model(cfg)
```

## The reason
The variable `ResNet50StagesTo4` is a global generator expression, so when we build the model secondly, the code executes to Line 82 `for stage_spec in stage_specs:` and `stage_specs` will return empty leading to do not add any stage. Finally, `self._freeze_backbone` try to freeze the backbone by executing `m = getattr(self, "layer" + str(stage_index))` firstly. At the moment, it will throw the AttributeError `AttributeError: 'ResNet' object has no attribute 'layer1'`.

I guess you want to define ResNet50StagesTo4 as the tuple, so I try to fix by add tuple type qualifier.

## The solution
Add the tuple type to `ResNet50StagesTo5`, `ResNet50StagesTo4`, `ResNet50FPNStagesTo5`, `ResNet101FPNStagesTo5`.

I do not know whether there are similar bug existing, so you need to review my solution. Thank you!

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