DDPCPU not woking with DDPPlugin(find_unused_parameters=True) in Ver 1.2.0 
🐛 Bug
when using accelerator="ddp_cpu" together with plugins=[DDPPlugin(find_unused_parameters=True)] to create a trainer, the trainer will cause the program tries to re-run its self (and recreate the trainer) and finally then failed at checking gpu devices.
Please reproduce using the BoringModel
    trainer = Trainer(
        max_epochs=1,
        gpus=0,
        accelerator="ddp_cpu",
        num_processes=4,
        plugins=[DDPPlugin(find_unused_parameters=True)],
    )
To Reproduce
Expected behavior
Environment

PyTorch Version (e.g., 1.0): 1.7.1
OS (e.g., Linux): osX
How you installed PyTorch (conda, pip, source): pip
Build command you used (if compiling from source):
Python version: 3.8.7
CUDA/cuDNN version: No
GPU models and configuration: gpus=None
Any other relevant information:

Additional context