[fix] remove unnecessary dp_size assert (#5351)

* fix: remove unnecessary assert

* test: add more 3d plugin tests

* fix: add warning

colossalai/booster/plugin/hybrid_parallel_plugin.py

 import ctypes
 import random
+import warnings
 from contextlib import contextmanager
 from functools import partial
 from types import MethodType
@@ -1134,7 +1135,12 @@ class HybridParallelPlugin(PipelinePluginBase):
                         tp_process_group=self.tp_group,
                     )
             else:
-                assert self.dp_size > 1, "Please use Zero when data parallel size is greater than 1."
+                if self.dp_size == 1:
+                    warnings.warn(
+                        "Use Zero Optimizer when data parallel size is 1 may introduce unnecessary overhead. "
+                        "If you are not intended to use cpu_offload, please consider set zero_stage=0."
+                    )
+
                 assert self.precision != "fp32", "Please set precision to 'fp16' or 'bf16' when using ZeRO."
                 optimizer = HybridParallelZeroOptimizer(
                     optimizer,

tests/test_booster/test_plugin/test_3d_plugin.py

@@ -118,6 +118,20 @@ def check_3d_plugin(init_method: str = "none", early_stop: bool = True):
 @parameterize(
     "test_args",
     [
+        {
+            "batch_size": 8,
+            "num_steps": 4,
+            "tp": 2,
+            "pp": 2,
+            "pp_style": "1f1b",
+            "num_model_chunks": 1,
+            "num_microbatches": 4,
+            "zero": 1,
+            "precision": "fp16",
+            "initial_scale": 1,
+            "max_length": 512,
+            "gradient_accumulation_step": 2,
+        },
         {
             "batch_size": 8,
             "num_steps": 4,