Add batch_first arg for attention.

[jonthegeek]

I haven't tested this; Rstudio crashed when I tried to build torch locally. Theoretically it's straightforward, but I'd definitely like to test it before I call this done. Any help is appreciated!

[dfalbel]

To build torch locally you will need to run source('tools/buildlantern.R'). THen devtools::load_all() should just work.

[jonthegeek]

I'm getting a bunch of errors like this: c++.exe: error: C:/rtools42/permissive: No such file or directory

Evidently some make vars are getting misinterpreted.

It may be time for me to finally learn about this WSL thing to run Linux on a Windows machine...

[dfalbel]

Hmm, I haven't seen those errors. Might be a problem with Rtools?
Or maybe you will need to install the visual studio c++ compiler . I'll review this PR tomorrow!

[jonthegeek]

I was able to install this branch with install_github, so it's specifically something about sourcing that makes rtools act up. I'll keep poking at it, but I can use this workaround to try to actually sort out tests tomorrow.

[dfalbel]

Fix #829