[keras] added GC callback

keras.py

 # from itertools import izip
+import gc
 from collections import OrderedDict, defaultdict
 import fnmatch
+
 import numpy as np
 import tensorflow as tf
 from tqdm import tqdm
@@ -597,6 +599,11 @@ class ModelLH(tf.keras.Model):
         return ret
 
 
+class GCCallback(tf.keras.callbacks.Callback):
+    def on_epoch_end(self, epoch, logs=None):
+        gc.collect()
+
+
 class TensorBoard(tf.keras.callbacks.TensorBoard):
     def __init__(self, *args, **kwargs):
         self.writer = kwargs.pop("writer")