[keras] gfeed: now uses list of validation callbacks and not only one

keras.py

@@ -90,7 +90,7 @@ class KFeed(object):
             **kwargs,
         )
 
-    def gfeed(self, src, batch_size, rng=np.random, auto_steps=np.max, validation=None, **kwargs):
+    def gfeed(self, src, batch_size, rng=np.random, auto_steps=np.max, val_callbacks=[], **kwargs):
         """
         Creates a generator for tf.keras' model.fit().
         Requires, that mean weights per process are equal:
@@ -113,14 +113,14 @@ class KFeed(object):
             workers=0,
             **kwargs,
         )
-        if validation is not None:
-            validation_copy = validation.copy()
+        for val_callback in val_callbacks:
+            val_callback_copy = val_callback.copy()
             ret.setdefault("callbacks", []).insert(
                 0,
-                validation_copy.pop("cls", CustomValidation)(
+                val_callback_copy.pop("cls", CustomValidation)(
                     **dict(
-                        zip(("x", "y", "sample_weight"), ret.pop("validation_data")),
-                        **validation_copy,
+                        zip(("x", "y", "sample_weight"), ret.get("validation_data")),
+                        **val_callback_copy,
                     )
                 ),
             )