[DSD] Add a test to verify FSDP lazy initialization case (#127069) (#127130)

* [DSD] Add a test to verify FSDP lazy initialization case (#127069)

Summary:
Distributed state_dict should not error out because the `model.state_dict()` will trigger FSDP to initialize.

Pull Request resolved: #127069
Approved by: https://github.com/wz337

* Add missing import get_optimizer_state_dict

---------

Co-authored-by: Andrey Talman <atalman@fb.com>

Author: fegin

test/distributed/checkpoint/test_state_dict.py

@@ -22,6 +22,7 @@
     _patch_model_state_dict,
     _patch_optimizer_state_dict,
     get_model_state_dict,
+    get_optimizer_state_dict,
     get_state_dict,
     set_model_state_dict,
     set_state_dict,
@@ -555,6 +556,19 @@ def _test_activation_ckpt_fqns_fsdp1(self, use_orig_params: bool) -> None:
 
         self.assertEqual(original_keys, new_keys)
 
+    @with_comms
+    @skip_if_lt_x_gpu(2)
+    def test_fsdp_root_not_initialized(self) -> None:
+        # This test verifies that FSDP root is not initialized but we should
+        # still be able to  get the state_dict without errors because
+        # fsdp_model.state_dict() will trigger the FSDP initialization.
+        device_mesh = init_device_mesh("cuda", (self.world_size,))
+        model = CompositeParamModel(device=torch.device("cuda"))
+        fsdp_model = FSDP(copy.deepcopy(model), device_mesh=device_mesh)
+        fsdp_optim = torch.optim.Adam(fsdp_model.parameters())
+        get_model_state_dict(fsdp_model)
+        get_optimizer_state_dict(fsdp_model, fsdp_optim)
+
 
 if __name__ == "__main__":
     run_tests()