Handle 3+ dimensional input in sequence_generator + nits

Summary: sequence_generator assumes that model input is 2d tensor of longs. But it can be something like 3d tensor of floats and we should be able to handle this as long as first dimension is batch size followed by source lengths.

Reviewed By: myleott

Differential Revision: D14420044

fbshipit-source-id: bf8b1e42ad1873f7b803c1a377b0af21648db015