fix move fp32 shards (#1604)

colossalai/zero/sharded_optim/sharded_optim_v2.py

@@ -288,6 +288,8 @@ class ShardedOptimizerV2(ColossalaiOptimizer):
             fp32_shards_used_cuda_margin_mem = 0
             for group in self.optim.param_groups:
                 for p in group['params']:
+                    if p.colo_attr.saved_grad.is_null():
+                        continue
                     shard_mem = self.master_params[p].payload.numel() * self.master_params[p].payload.element_size()
                     if fp32_shards_used_cuda_margin_mem + shard_mem < fp32_shards_available_cuda_margin_mem:
                         colo_model_data_tensor_move_inline(self.master_params[p], torch.cuda.current_device())