fixed torch 0.4.0 , "RuntimeError: Expected object of type torch.cuda… (#393)

Summary:
….LongTensor but found type torch.cuda.FloatTensor for argument #3 'index' " error

in the torch.__version__ == 0.4.0 ,
new_order = torch.arange(bsz).view(-1, 1).repeat(1, beam_size).view(-1)
will return a float dtype Tensor, when exec the "line 321: fairseq/fairseq/models/fconv.py " will throw a RuntimeError
Pull Request resolved: https://github.com/pytorch/fairseq/pull/393

Differential Revision: D13276496

Pulled By: myleott

fbshipit-source-id: e7986246fbe2c79fff61bcab0e5bec9dd63e0afd