Fix truncated_bptt_steps hiddens detach() and improve docs

[gtauzin]

What does this PR do?

This PR was discussed on Slack with @awaelchli.
It fixes the detach() on hiddens in the case it is a tuple of Tensors (typically, for nn.LSTM).
It reverts the documentation change that I pushed on #7846 as it turned out to be incorrect.
It also proposes a documentation improvement explaining a bit more what (I understood) happens behind the scenes and stresses the importance of using it along with batch_first=True. Please let me know if this is correct!

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Did you have fun?

Make sure you had fun coding 🙃

[gtauzin]

Could I get any suggestions wrt where I should add my test?

[codecov]

Codecov Report

Merging #8145 (bc858af) into master (3e6f884) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #8145    +/-   ##
=======================================
- Coverage      93%     88%    -5%     
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  Files         212     212            
  Lines       13679   13693    +14     
=======================================
- Hits        12720   12050   -670     
- Misses        959    1643   +684     

[awaelchli]

great improvement!

[pep8speaks]

Hello @gtauzin! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-01 16:28:20 UTC

[gtauzin]

I believe it is ready. Not that I could not run the test locally on my laptop. The make test fails for a rather high number of tests unrelated to my change and hangs without finishing.

[awaelchli]

Thanks @gtauzin <3

[gtauzin]

You're welcome @awaelchli. Any idea when the release including this change will be out?

[awaelchli]

@gtauzin since this is a bugfix, it will go into next week's patch release and/or RC release, given that it is merged until then :)