import keras
import numpy as np

model = keras.Sequential([
    keras.Input(shape = (20,)),
    keras.layers.Embedding( input_dim = 10,
                           output_dim = 5,
                            mask_zero = True
                            ),

    keras.layers.Bidirectional(keras.layers.LSTM(units = 10, return_sequences = True )),
    keras.layers.TimeDistributed(keras.layers.Dense(units = 5, activation = "softmax")  ),#not works with mask_zero
])
model.compile(
    optimizer="adam",
    loss="binary_crossentropy",
    metrics=["accuracy"],
)
X_train = np.random.uniform(1,10,size = (50,20))
Y_train = np.random.randint(1,2,size = (50,20,5))

model.fit(X_train, Y_train, epochs = 2, batch_size = 8)