Add extra check for safety.
Elemwise now special case the code when all inputs are contiguous.

Do not work yet when the inputs is fortran, as the outputs is always c contig for now.
Elemwise output is now fortran when all inputs are fortran.

This speed up hard_sigmoid and ultra_fast_sigmoid by 2x.

We ignore input that are broadcasted scalar when checking for fortran
inputs.
Fix indent (not wrong code) and better comment following code review.
Add doc file to the toc
use simpler elemwise code for [broadcasted] scalar.
Merge pull request #1393 from nouiz/fcontig

Elemwise faster when c or f contig inputs