Added Union import to fix import error

app.py

@@ -156,6 +156,7 @@ import tensorflow as tf
 import numpy as np
 import tensorflow.keras.backend as K
 from tensorflow.keras.metrics import Metric
+from typing import Union
 FloatTensorLike = Union[tf.Tensor, float, np.float16, np.float32, np.float64]
 AcceptableDTypes = Union[tf.DType, np.dtype, type, int, str, None]