[Tiny PR] [Fix] Use random number generator in weighted mode of select

gflownet/utils/buffer.py

@@ -292,26 +292,25 @@ def select(
             - weighted: data points are sampled with probability proportional to their
               score.
 
-        Args
-        ----
+        Parameters
+        ----------
         data_dict : dict
             A dictionary with samples (key "x") and scores (key "energy" or "rewards").
-
         n : int
             The number of samples to select from the dictionary.
-
         mode : str
             Sampling mode. Options: permutation, weighted.
-
         rng : np.random.Generator
-            A numpy random number generator, used for the permutation mode. Ignored
-            otherwise.
+            A numpy random number generator, used for the permutation and weighted
+            modes. If None (default), a generator with a random seed is used.
 
         Returns
         -------
         list
             A batch of n samples, selected from data_dict.
         """
+        if rng is None:
+            rng = np.random.default_rng()
         if n == 0:
             return []
         samples = data_dict["x"]
@@ -320,7 +319,6 @@ def select(
         if isinstance(samples, dict):
             samples = list(samples.values())
         if mode == "permutation":
-            assert rng is not None
             indices = rng.choice(
                 len(samples),
                 size=n,
@@ -339,7 +337,7 @@ def select(
             # need to keep its values only
             if isinstance(scores, dict):
                 scores = np.fromiter(scores.values(), dtype=float)
-            indices = np.random.choice(
+            indices = rng.choice(
                 len(samples),
                 size=n,
                 replace=False,