fix ShardedTensor.gather when shard is empty (#110962)
Summary:
current ShardedTensor.gather is not working as expectation when the shard is empty on any rank
The root cause is identified that when a sharded tensor has no placement on a specific rank, the metadata doesn't include that rank's placement which introduces KeyError in : ```shard_offset = shard_placement[shard. Metadata][1]```
It's fixed by adding an empty tensor check.
Test Plan:
before change:
after change:
Differential Revision: D50114085
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110962
Approved by: https://github.com/wz337
