num_future not used

`num_future` is included as an argument but not actually used.

https://github.com/traja-team/traja/blob/659c8e0978042d561f8e13a4b0786d28c2ed7a41/traja/models/predictive_models/lstm.py#L31

@Saran-nns can you please remove or implement this?