fix: use model_id instead of self.model_id when output_model_name is None in train_model

Author: Jean-Baptiste Oger

opensearch_py_ml/ml_models/sentencetransformermodel.py

@@ -421,7 +421,7 @@ def train_model(
         if model_id is None:
             model_id = self.model_id
         if output_model_name is None:
-            output_model_name = str(self.model_id.split("/")[-1] + ".pt")
+            output_model_name = str(model_id.split("/")[-1] + ".pt")
 
         # declare variables before assignment for training
         corp_len = []