added mkl to lib torch as option, and fbgemm as feature

[EvilMcStevil]

• Changes comply with the maintainer guide.
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[dg0yt]

AFAICT this is the port which is to select the BLAS implementation. It is not a per-port choice.

[EvilMcStevil]

i noticed that, but something has changed since libtorch got accepted recently, and caused the linker to fail due to CRT/static/dynamic library linking. I suspect it is the blas implementation but trying to prove it is a multiple day long compile fest. Im trying to avoid fiddling with that can of worms until i have proven out other things.
Libtorchs prefered blas implementation is mkl and it will use that if found regardless of any others available, im just trying to prove compilation first to work out which platforms it correctly support before i just add it as a feature.

What does confuse me is why the linking has just started failing, without being picked up by the pipeline

[azure-pipelines]

Commenter does not have sufficient privileges for PR 47814 in repo microsoft/vcpkg