[optim] Set defaults to foreach, NOT fused (#95241) (#95415)
Rolling back the default change for Adam and rectifying the docs to reflect that AdamW never defaulted to fused.
Since our fused implementations are relatively newer, let's give them a longer bake-in time before flipping the switch for every user.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95241
Approved by: https://github.com/ngimel
