Fixed the issue that universal checkpoint cannot be loaded for stage3 when world size expansion.

[zhengchenyu]

When the world size expands from 2 to 4, then convert to universal checkpoint, and load from universal checkpoint.
The new rank, for example, rank3 will load model file zero_pp_rank_3_mp_rank_00_model_states.pt. But this file was not produced during the last execution.
For stage3, just load the first file, that is zero_pp_rank_0_mp_rank_00_model_states.
The existing unit test TestZeROUniversalCheckpointDP::test_dp_world_size_2to4 can verify this problem.