Add prepacked weights container to subgraphs (#17671)
### Description
Adds prepacked weights container to model subgraphs.
### Motivation and Context
Allows for initializer sharing when the initializers are located in
subgraphs. I encountered this bug when attempting to share weights
between T5 BeamSearch models where the shareable initializers are
located in the encoder and decoder subgraphs and it failed to reduce
memory usage.
