[DDP] Perform input casting in pre forward (#100131)
This is so that replicate can also have the feature to cast its
inputs, which it currently does not. Next diff will change replicate pre hook
to support this.
Differential Revision: [D45335179](https://our.internmc.facebook.com/intern/diff/D45335179/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100131
Approved by: https://github.com/zhaojuanmao
