[sharded_tensor] fix typing issue for placement (#63426)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63426
placement should either be a string or a _remote_device, this fixes the type to match the behaviors
ghstack-source-id: 136041125
Reviewed By: pritamdamania87
Differential Revision: D30379702
fbshipit-source-id: 34e226494240923b433e3a39cc08c84d42cdad6b
