Are you still able to load the checkpoint? DataParallel just hides the actual model away under the attribute module. So you can do something like this:
my_model = ModelClass(...)
my_model = DataParallel(my_model, ...)

state_dict = torch.load(...)
my_model.load_state_dict(state_dict)

actual_model = my_model.module
new_state_dict = actual_model

Haven’t tested this, but I bet something like this would work as well:
state_dict = torch.load(...)

new_state_dict = {}
for k, v in state_dict.items():
    k = k.replace('module.', '')  # Each weight k will be prefix with the word "module"
    new_state_dict[k] = v

model = SomeModel(...)
model.load_state_dict(new_state_dict)