Move weight offload inside backend construction functor (#18385)

Move weight offload inside backend construction functor (#18385)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18385

By moving the weight offload into the backend initialization function, we can instantiate the backend once by creating the OnnxifiOp once and then clean up the parameter workspace. And we need to keep hold of that instantiated net (OnnxifiOp) without cleaning it. Subsequent ctor of OnnxifiOp of the same model will hit the cached backend and they will not look into weight offloading, which is safe as the weight is already gone.

Reviewed By: ipiszy

Differential Revision: D14590379

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