fix: avoid IndexError in BF16_Optimizer.destroy() when using DummyOptim (#7763)

fix: avoid IndexError in BF16_Optimizer.destroy() when using DummyOptim

Short-circuit BF16_Optimizer.destroy() if using_real_optimizer is False.
When initialized with optimizer=None (DummyOptim), bf16_groups remains
empty, causing an IndexError when accessing it in destroy().

Resolves #7752

Author: nathon-lee

deepspeed/runtime/bf16_optimizer.py

@@ -103,6 +103,8 @@ def __init__(self,
         see_memory_usage('end bf16_ optimizer', force=True)
 
     def destroy(self):
+        if not self.using_real_optimizer:
+            return
         for i, _ in enumerate(self.optimizer.param_groups):
             for p in self.bf16_groups[i]:
                 if getattr(p, '_hp_mapping', None):