Make Dints TensorRT Convertable

Is your feature request related to a problem? Please describe.
TensorRT is a SDK for high-performance deep learning inference. For the trained pytorch models, Torch-TensorRT is the most convenient way to apply the TensorRT acceleration. Therefore, we are trying to accelerate DiNTS trought this way.

However, currently the DiNTS net structure in MONAI cannot be converted through Torch-TensorRT. After having a discussion with the Torch-TensorRT team, this failed convert is caused by the torch.nn.ModuleDict module in the network like this one, which is not supported by Torch-TensorRT.

Describe the solution you'd like
As the Torch-TensorRT team suggest, there are two ways that are worth to try:

1. unpack torch.nn.ModuleDict and lay out the modules in a flat manner during inference phase.
2. replace it with a Python level dictionary

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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