Update pybind11 usage for sub plugin classes

[tp-nan]

the following test would be failed in some cases:

python3 -m torch2trt.test --name=group  

TypeError: add_plugin_v2(): incompatible function arguments. The following argument types are supported:
    1. (self: tensorrt.tensorrt.INetworkDefinition, inputs: List[tensorrt.tensorrt.ITensor], plugin: tensorrt.tensorrt.IPluginV2) -> tensorrt.tensorrt.IPluginV2Layer

Invoked with: <tensorrt.tensorrt.INetworkDefinition object at 0x7f270e6e2a40>, [<tensorrt.tensorrt.ITensor object at 0x7f2726321d18>], <torch2trt.plugins.GroupNormPlugin object at 0x7f2726321f80>

see

#https://github.com/NVIDIA-AI-IOT/torch2trt/pull/656#issuecomment-970491423 #https://github.com/NVIDIA-AI-IOT/torch2trt/pull/656#issuecomment-1010937335

This PR try to fix this issue.