message: [Keras]: cover input layer in sequential model
diff: #
  from flexflow.keras.models import Sequential
  -from flexflow.keras.layers import Conv2D, MaxPooling2D, Flatten, Dense, Activation
  +from flexflow.keras.layers import Conv2D, MaxPooling2D, Flatten, Dense, Activation, Input
  import flexflow.keras.optimizers
  from flexflow.keras.datasets import mnist
  from flexflow.keras.callbacks import Callback, VerifyMetrics
  @@ -38,7 +38,8 @@ def top_level_task():
  y_train = np.reshape(y_train, (len(y_train), 1))
  print("shape: ", x_train.shape, x_train.__array_interface__["strides"])
  - layers = [Conv2D(filters=32, input_shape=(1,28,28), kernel_size=(3,3), strides=(1,1), padding=(1,1), activation="relu"),
  + layers = [Input(shape=(1, 28, 28), dtype="float32"),
  + Conv2D(filters=32, kernel_size=(3,3), strides=(1,1), padding=(1,1), activation="relu"),
  Conv2D(filters=64, kernel_size=(3,3), strides=(1,1), padding=(1,1), activation="relu"),
  MaxPooling2D(pool_size=(2,2), strides=(2,2), padding="valid"),
  Flatten(),
  