Lengths are not passed to the RNN Encoder

[eugene-kharitonov]

Passing an-already computed lengths of a message is a micro-optimization.

Apparently, it is lost in here: https://github.com/facebookresearch/EGG/blob/63a9e0dae9cb72e63d9191c633d346747e45ce5b/egg/core/reinforce_wrappers.py#L300

Thanks @robertodessi for spotting!