Need to tune performance for MXNet & TensorFlow for seq2seq

[astonzhang]

http://preview.d2l.ai.s3-website-us-west-2.amazonaws.com/d2l-en/master/chapter_recurrent-modern/seq2seq.html http://preview.d2l.ai.s3-website-us-west-2.amazonaws.com/d2l-en/master/chapter_attention-mechanisms/bahdanau-attention.html http://preview.d2l.ai.s3-website-us-west-2.amazonaws.com/d2l-en/master/chapter_attention-mechanisms/transformer.html

We need to tune performance for MXNet & TensorFlow to obtain similar performance of PyTorch for each section, such as learning rate & max_epochs.

[AnirudhDagar]

http://preview.d2l.ai.s3-website-us-west-2.amazonaws.com/d2l-en/master/chapter_attention-mechanisms/bahdanau-attention.html was fixed in #2104. seq2seq and transformers for TF still need fixing.