Unwrap PipelinedPostproc modules when detaching model from pipeline (meta-pytorch#2822)

Summary:
Pull Request resolved: meta-pytorch#2822

Ensure that we unwrap `PipelinedPostproc` modules when calling `detach()`. Otherwise these modules will return the cached results after unwrapping

Reviewed By: che-sh

Differential Revision: D71150731

fbshipit-source-id: f7bb172afca7f8f511fd7b96decd5e749cc5b2c5

Author: sarckk