Implement standard attention and self-attention module

[albertz]

Via cum_concat etc.
rwth-i6/returnn#391
rwth-i6/returnn#656
rwth-i6/returnn#589
rwth-i6/returnn#590

[albertz]

For this, I think we really should clarify the dim tags first (#17).

[albertz]

I implemented some first variants. Although they are not really tested, but probably we want to test them together with #55 or #58.

If anything is broken, or incomplete, or needs to be adopted for #17, I think we should just open new issues.

[albertz]

Just for reference, this lacks positional encoding so far.

Some relevant discussion:
pytorch/pytorch#24826
allenai/allennlp#3398

[albertz]

See also: Pytorch MultiheadAttention, via pytorch/pytorch#18334