Update WMT16 standard hparams

[pramodkaushik]

"num_encoder_layers" and "num_decoder_layers" are set in nmt.create_hparams() (Line 595 in nmt.py), but not updated after calling nmt.misc_utils.maybe_parse_standard_hparams() (Line 497 in nmt.py).

Fixes the issue (#264) where loading the pretrained checkpoint was resulting in an error.

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[pramodkaushik]

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