Update to tensorflow 2.0

[SrMouraSilva]

A list of improvements for this implementation. Part of this list has been implemented in https://colab.research.google.com/drive/1FohOZGI4Knw64EJuMgetqFbu0_cTpqRz

• Use tensorflow datasets;
• Matrix multiplication with @ operator;
• Update some methods calls;
• Use tf.nn.softplus instead manual definition;
• [Not implemented] Use tf.function to autograph;
• Use tf.gradient_tape.