[hotfix] fix the bug that large tensor exceed the maximum capacity of…

… TensorBucket (hpcaitech#5879)

colossalai/zero/low_level/low_level_optim.py

@@ -549,6 +549,13 @@ def step(self, closure=None):
                 working_param = real_working_params[group_id][idx]
                 param_to_gather = master_param.to(device).to(self._dtype)
                 pg = self.param_to_pg[working_param]
+                if param_to_gather.numel() > self.pg_to_tensor_bucket[pg].max_size:
+                    buffer_tensor = torch.empty_like(
+                        torch.cat([param_to_gather for _ in range(dist.get_world_size(pg))])
+                    )
+                    dist.all_gather_into_tensor(buffer_tensor, param_to_gather, pg)
+                    working_param.data.copy_(buffer_tensor[: working_param.numel()].reshape_as(working_param))
+                    continue
                 try:
                     self.pg_to_tensor_bucket[pg].add_to_bucket(param_to_gather, write_back_tensor=working_param)
                 except RuntimeError: