Replace deprecated botorch method to remove warning

optuna/terminator/improvement/gp/botorch.py

@@ -3,6 +3,7 @@
 from typing import Optional
 
 import numpy as np
+from packaging import version
 
 from optuna._imports import try_import
 from optuna.distributions import _is_distribution_log
@@ -16,15 +17,20 @@
 
 
 with try_import() as _imports:
-    from botorch.fit import fit_gpytorch_model
+    import botorch
     from botorch.models import SingleTaskGP
     from botorch.models.transforms import Normalize
     from botorch.models.transforms import Standardize
     import gpytorch
     import torch
 
+    if version.parse(botorch.version.version) < version.parse("0.8.0"):
+        from botorch.fit import fit_gpytorch_model as fit_gpytorch_mll
+    else:
+        from botorch.fit import fit_gpytorch_mll
+
 __all__ = [
-    "fit_gpytorch_model",
+    "fit_gpytorch_mll",
     "SingleTaskGP",
     "Normalize",
     "Standardize",
@@ -61,7 +67,7 @@ def fit(
 
         mll = gpytorch.mlls.ExactMarginalLogLikelihood(self._gp.likelihood, self._gp)
 
-        fit_gpytorch_model(mll)
+        fit_gpytorch_mll(mll)
 
     def predict_mean_std(
         self,