add version check about BN freeze

tensorpack/models/batch_norm.py

@@ -9,6 +9,7 @@ from tensorflow.python.training import moving_averages
 
 from ..utils import logger
 from ..tfutils.tower import get_current_tower_context
+from ..tfutils.common import get_tf_version_number
 from ..tfutils.collection import backup_collection, restore_collection
 from .common import layer_register, VariableHolder
 
@@ -135,6 +136,9 @@ def BatchNorm(x, use_local_stat=None, decay=0.9, epsilon=1e-5,
             xn = tf.squeeze(xn, [1, 2])
     else:
         if ctx.is_training:
+            assert get_tf_version_number() >= 1.4, \
+                "Fine tuning a BatchNorm model with fixed statistics is only " \
+                "supported after https://github.com/tensorflow/tensorflow/pull/12580 "
             if ctx.index == 0:  # only warn in first tower
                 logger.warn("[BatchNorm] Using moving_mean/moving_variance in training.")
             # Using moving_mean/moving_variance in training, which means we