docs: explain first-iteration training skip

docs/input.md

@@ -52,12 +52,19 @@ This section defines how a model is trained.
 	"numb_models" : 4,
 	"config" : {},
 	"template_script" : "/path/to/the/template/input.json",
+	"init_models_paths" : [
+		"/path/to/model.000.pb",
+		"/path/to/model.001.pb",
+		"/path/to/model.002.pb",
+		"/path/to/model.003.pb"
+	],
 	"_comment" : "all"
 }
 ```
 The `"type" : "dp"` tell the traning method is {dargs:argument}`"dp" <train>`, i.e. calling [DeePMD-kit](https://github.com/deepmodeling/deepmd-kit) to train DP models.
 The `"config"` key defines the training configs, see {ref}`the full documentation<train[dp]/config>`.
 The {dargs:argument}`"template_script" <train[dp]/template_script>` provides the template training script in `json` format.
+When {dargs:argument}`"init_models_paths" <train[dp]/init_models_paths>` supplies one model per committee member and those models were already trained on the initial dataset, DPGEN2 automatically skips the iteration-zero training command. It records the generated training script and a skip message, then passes the supplied models directly to exploration. Training resumes after labeling produces iteration data. Finetuning requested with `"do_finetune": true` is never skipped.
 
 
 ### Exploration

dpgen2/entrypoint/args.py

@@ -85,7 +85,11 @@ def dp_train_args():
     doc_numb_models = "Number of models trained for evaluating the model deviation"
     doc_config = "Configuration of training"
     doc_template_script = "File names of the template training script. It can be a `List[str]`, the length of which is the same as `numb_models`. Each template script in the list is used to train a model. Can be a `str`, the models share the same template training script. "
-    doc_init_models_paths = "the paths to initial models"
+    doc_init_models_paths = (
+        "Paths to initial models. When these models already represent the "
+        "initial dataset, DPGEN2 reuses them and skips training in iteration "
+        "zero because no iteration-generated data exists yet."
+    )
     doc_init_models_uri = "The URI of initial models"
     doc_optional_files = "Optional files for training"
 

tests/op/test_run_dp_train.py

@@ -945,7 +945,8 @@ def test_update_input_dict_v2_empty_list(self):
         )
         self.assertDictEqual(odict, self.expected_odict_v2)
 
-    def test_exec_v2_empty_list(self):
+    @patch("dpgen2.op.run_dp_train.run_command")
+    def test_exec_v2_empty_list(self, mocked_run):
         config = self.config.copy()
         config["init_model_policy"] = "no"
 
@@ -989,6 +990,7 @@ def test_exec_v2_empty_list(self):
             jdata = json.load(fp)
             self.assertDictEqual(jdata, self.expected_odict_v2)
         self.assertEqual(Path(out["model"]).read_text(), "this is init model")
+        mocked_run.assert_not_called()
 
         os.remove(self.init_model)