Update Language Modeling with nn.Transformer and TorchText tutorial

[subramen]

Fixes batchify to return tensor of shape [batch_size, seq_len] (instead of [seq_len, batch_size]).

Closes #1841

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[svekars]

This PR just constantly fails with:

Dec 09 22:10:55 torch.cuda.OutOfMemoryError: CUDA out of memory. Tried to allocate 1.45 GiB (GPU 0; 7.43 GiB total capacity; 6.24 GiB already allocated; 537.62 MiB free; 6.25 GiB reserved in total by PyTorch) If reserved memory is >> allocated memory try setting max_split_size_mb to avoid fragmentation.  See documentation for Memory Management and PYTORCH_CUDA_ALLOC_CONF

@malfet any thoughts?

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