fix: use explicit session in PaLM2TextGenerator (#651)

This is to avoid running into conflict with the global session which may
have different options (e.g. location, connection etc.) set.

Author: shobsi

bigframes/ml/llm.py

@@ -144,7 +144,7 @@ def _from_bq(
         kwargs: dict = {}
         last_fitting = model.training_runs[-1]["trainingOptions"]
 
-        dummy_text_generator = cls()
+        dummy_text_generator = cls(session=session)
         for bf_param, _ in dummy_text_generator.__dict__.items():
             bqml_param = _BQML_PARAMS_MAPPING.get(bf_param)
             if bqml_param in last_fitting: