TBLIS executor

[janbrandejs]

currently, when someone wants to use TAPP with TBLIS, they have to install TBLIS themselves (in case of make) or use cmake with -DENABLE_TBLIS=1.
Then to have TBLIS running as backend for all contractions, they "simply" change
int ex = 1;
to
ex = 2;

in create_executor in src/tapp/executor.c

I suggested to enable this automatically based on some cmake variable (see comment in the code), but ENABLE_TBLIS is probably not the best choice, as we need backend choosing logic separate, because TBLIS can normally only served for verifying correctness in test.cpp. How would you approach this? @devinamatthews @SpaceyLake @evaleev ? Should we include a compile time constant DEFAULT_BACKEND/DEFAULT_EXECUTOR?

[devinamatthews]

Wouldn't it be easier to flip things around and put the tapp interface in tblis? If you take what you have (however rough) and make a pr on tblis that will jump-start things.

[evaleev]

TAPP is meant to be a BLAS-like API, so I agree with @devinamatthews . TAPPorg/reference-implementation is the reference implementation of the TAPP API + dox + tests that can use TBLIS (directly) for validation. It should not provide TBLIS-based TAPP API. The latter should be in TBLIS directly. This will simplify the build harness etc.

What TAPP ref impl now tries to do is TAPPorg/trampoline a la BLAS trampoline, 1-stop provider of TAPP API that can choose the backend programmatically at runtime. This will hopefully be relatively simple to implement if every provider of TAPP API has a proper CMake harness.