Fix numpy incompatibility change for PSRO's joint to marginal probability function. See chat in #1148

Fixes: #1148.
PiperOrigin-RevId: 600726087
Change-Id: If5252c931874283f5fea4acb7885f90d4c83f8c0

Author: DeepMind Technologies Ltd

open_spiel/python/algorithms/psro_v2/meta_strategies.py

@@ -95,21 +95,18 @@ def renormalize(probabilities):
 
 
 def get_joint_strategy_from_marginals(probabilities):
-  """Returns a joint strategy matrix from a list of marginals.
+  """Returns a joint strategy tensor from a list of marginals.
 
   Args:
     probabilities: list of probabilities.
 
   Returns:
-    A joint strategy from a list of marginals.
+    A flat joint strategy from a list of marginals.
   """
-  probas = []
-  for i in range(len(probabilities)):
-    probas_shapes = [1] * len(probabilities)
-    probas_shapes[i] = -1
-    probas.append(probabilities[i].reshape(*probas_shapes))
-  result = np.prod(probas)
-  return result.reshape(-1)
+  res = np.ones((1,), dtype=np.float64)
+  for prob in probabilities:
+    res = res[..., None] @ np.asarray(prob).reshape((1,) * res.ndim + (-1,))
+  return res.reshape(-1)
 
 
 def nash_strategy(solver, return_joint=False):