Fix bug in logistic regression benchmark.

Programs/Source/logreg.mpc

@@ -9,6 +9,7 @@ cfix.set_precision(16, 31)
 
 dim = int(program.args[1])
 batch = int(program.args[2])
+ml.Layer.back_batch_size = batch
 
 try:
    n_iterations = int(program.args[3])