Update ensemble.py

[UditKaushish]

while running this line "from deepstack.ensemble import DirichletEnsemble" error is cannot import name 'to_categorical' from 'keras.utils' (/usr/local/lib/python3.7/dist-packages/keras/utils/init.py) if we use "from tensorflow.keras.utils import to_categorical" in place of "from keras.utils import to_categorical" then this error will be resolved.