Fix precision issue with expansion that prefers 'probs' over 'logits' (#18614)

Fix precision issue with expansion that prefers 'probs' over 'logits' (#18614)

Summary:
I have experienced that sometimes both were in `__dict__`, but it chose to copy `probs` which loses precision over `logits`. This is especially important when training (bayesian) neural networks or doing other type of optimization, since the loss is heavily affected.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18614

Differential Revision: D14793486

Pulled By: ezyang

fbshipit-source-id: d4ff5e34fbb4021ea9de9f58af09a7de00d80a63