[dtensor] Relax condition for _split_tensor() (#101218)
When tensor.size(self.dim) < num_chunks, we will fill empty chunk with empty tensor (https://github.com/pytorch/pytorch/pull/98722). Therefore, we no longer needs this assert.
For example, when sharding a tensor with 1 element on 2 ranks along dim 0, results would be as follows:
```
rank:0, dtensor:DTensor(local_tensor=tensor([0.4963], device='cuda:0'), device_mesh=DeviceMesh:([0, 1]), placements=[Shard(dim=0)])
rank:1, dtensor:DTensor(local_tensor=tensor([], device='cuda:1'), device_mesh=DeviceMesh:([0, 1]), placements=[Shard(dim=0)])
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101218
Approved by: https://github.com/wanchaol
