[PT-D][Sharded Tensor] new init api for local tensor and sharding spec auto inference (#72733)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72733
To improve the perf cost due to communication in the process of init the sharded tensor. There are two changes in this PR/diff:
1. We create a new API named `_init_from_local_tensor` so that if we have only one local tensor, we can initiate a sharded tensor directly from it. (GH issue: https://github.com/pytorch/pytorch/issues/72092)
2. We create a new API to infer the sharding spec from global meta data, so we don't have to manually set the sharding spec when it's not `EnumerableShardingSpec`. (GH issue: https://github.com/pytorch/pytorch/issues/67244)
ghstack-source-id: 149229259
Test Plan: CI
Reviewed By: wanchaol
Differential Revision: D34132739
fbshipit-source-id: 3a60135761bcc19d6020b6c45cb2979869645ce6
(cherry picked from commit af569325e2794309a4a86e51749642a062a25f6e)
