Add defence for offload_states and reload_states w/o optimizer (deepspeedai#7211)

When the optimizer is not specified, the optimizer will be type
`DeepSpeedZeRoOffload` instead of `DeepSpeedZeroOptimizer_Stage3` (e.g.
for ZeRO-3 pure inference), while `DeepSpeedZeRoOffload` hasn't
implemented methods `reload_states` and `offload_states`.

https://github.com/deepspeedai/DeepSpeed/blob/56005d2b256eb81a88cba0a1984375f9663a3110/deepspeed/runtime/engine.py#L1684-L1707

```log
  File "deepspeed/runtime/engine.py", line 3904, in offload_states
    self.optimizer.offload_states(include=include, device=device, pin_memory=pin_memory, non_blocking=non_blocking)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'DeepSpeedZeRoOffload' object has no attribute 'offload_states'
```

In addition, deepspeedai#6855 seems to
forget removing the check for `assert not self.zero_offload_param()`, as
suggested by
deepspeedai#6833 (comment),
it returns None when offload_param is not given, and the newly added
assertions have already covered these cases. This PR also removed this
old check.

Signed-off-by: Hollow Man <hollowman@opensuse.org>
Signed-off-by: Max Kovalenko <mkovalenko@habana.ai>

Author: HollowMan6

deepspeed/runtime/engine.py

@@ -3894,7 +3894,9 @@ def offload_states(self,
         param_offload_config = self.zero_offload_param()
         assert param_offload_config is None or param_offload_config.device == OffloadDeviceEnum.none, "Moving states across devices is not supported for offloaded parameters."
 
-        assert not self.zero_offload_param(), "Moving states across devices is not supported for offloaded parameters."
+        assert not isinstance(
+            self.optimizer,
+            DeepSpeedZeRoOffload), "Moving states across devices is not supported without an optimizer."
 
         if device == OffloadDeviceEnum.none:
             logger.warning("No device specified for offloading states.")
@@ -3913,4 +3915,9 @@ def reload_states(self, non_blocking: bool = False) -> None:
         """
         assert self.zero_optimization_stage(
         ) == ZeroStageEnum.weights, "Moving buffers back is supported only for ZeRO stage 3."
+
+        assert not isinstance(
+            self.optimizer,
+            DeepSpeedZeRoOffload), "Moving states across devices is not supported without an optimizer."
+
         self.optimizer.reload_states(non_blocking=non_blocking)