Tensorflow doc changes on loss output size
Author: janjitse
Pull request: yes

# What does this PR do?

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Fixes #9771, by changing the documentation to correctly state the size of the output loss function.

I did not change the documentation for TFSeq2SeqQuestionAnsweringModelOutput and TFSeq2SeqSequenceClassifierOutput, as I could not find any code using this, so I was unsure what the correct output size would be.

I also fixed a few instances where I found the documentation referring to torch.LongTensor when tf.tensor should be used.


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## Who can review?
@jplu 


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