Warn when switching to eval with BN but no stats.

Author: lucasb-eyer

DeepFried2/layers/BatchNormalization.py

@@ -2,6 +2,7 @@
 from DeepFried2.utils import flatten
 
 import numpy as _np
+from warnings import warn as _warn
 
 
 class BatchNormalization(df.Module):
@@ -84,6 +85,10 @@ def evaluate(self):
         self.Winf.set_value(self.W.get_value() / _np.sqrt(self.buf_var.get_value() + self.eps))
         self.binf.set_value(self.b.get_value() - self.Winf.get_value() * self.buf_mean.get_value())
 
+        # This check saved me from WTF'ing countless times!
+        if self.buf_count.get_value() == 0:
+            _warn("You're switching a BN-net to eval mode without having collected any statistics, that can't go well!")
+
     def __getstate__(self):
         regular = df.Module.__getstate__(self)
         return [buf.get_value() for buf in (self.buf_mean, self.buf_var, self.buf_count)] + regular