Add Deepspeed Zero 3 MiCS support (Issues #20378)

Author: hehepig4

src/lightning/pytorch/strategies/deepspeed.py

@@ -521,7 +521,7 @@ def model_sharded_context(self) -> Generator[None, None, None]:
         import deepspeed
 
         self._init_config_if_needed()
-
+        assert self.config is not None
         # If detect 'mics_shard_size'>0 in config['zero_optimization'], alter to use deepspeed.zero.MiCS_Init()
         # https://deepspeed.readthedocs.io/en/latest/zero3.html#mics-configurations
         #! default deepspeed 0.9.0 is not compatible