Fix Attention in seq2seq_translation_tutorial AttnDecoderRNN

[QasimKhan5x]

Fixes #130

Description

• Changes AttnDecoderRNN by replacing the previous attention mechanism with the method used by Bahdanau et al.
• Changes the .png diagram that depicts the model to reflect the new one
• Minor fixes like replacing encoder1 and attn_decoder1 with encoder and attn_decoder

Also, in the current tutorial, the visualizations clearly show wrong usage of attention. By merging this pull request, the correct visual display of feature importance via attention will be shown.

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

cc @pytorch/team-text-core @Nayef211

[pytorch-bot]

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[Nayef211]

Thanks for the updated diagram @QasimKhan5x