make sure weights always sum up to 1, fix #4369 (#4370)

* make sure weights always sum up to 1, fix #4369

* always normalize self.weights in update_weights_beta

* rephrase

Author: basnijholt

pymc3/smc/smc.py

@@ -177,6 +177,8 @@ def update_weights_beta(self):
         self.log_marginal_likelihood += logsumexp(log_weights_un) - np.log(self.draws)
         self.beta = new_beta
         self.weights = np.exp(log_weights)
+        # We normalize again to correct for small numerical errors that might build up
+        self.weights /= self.weights.sum()
 
     def resample(self):
         """Resample particles based on importance weights."""