Fix bug from `transformer.ipynb` in `/experiments`

[A-baoYang]

Hi, thank you so much for open sourcing your project, it helps me a lot!!
Here I just have one simple bug to report:

---

Source: 3rd code cell from notebook

• Original:

outputs = Transformer(
    vocab_size, 
    model_dim, 
    n_heads=2, 
    encoder_stack=2,
    decoder_stack=2, 
    feed_forward_size=50
)([encoder_inputs, decoder_inputs])

After ran the notebook and traced your code (deep_recommenders/keras/models/nlp/Transformer.py),
the input of Transformer class object needs 2 parameters (encoder_inputs, decoder_inputs), it should not be a list.

• Revised:

outputs = Transformer(
    vocab_size, 
    model_dim, 
    n_heads=2, 
    encoder_stack=2,
    decoder_stack=2, 
    feed_forward_size=50
)(encoder_inputs, decoder_inputs)

[SharpCX]

We have fix the issues, thx.