[Kernel] Add GPU architecture guards to the CUTLASS w8a8 kernels to reduce binary size

[tlrmchlsmth]

This PR wraps the CUTLASS kernel definitions in order to guard against compilation on architectures that will never use a particular kernel. The purpose of this is to reduce the size of the compiled binary. Each CUTLASS kernel is defined and optimized for specific GPU architectures, but each kernel is compiled for every arch defined in CUDA_SUPPORTED_ARCHS.

The normal way to deal with this is to look at the macro __CUDA_ARCH__. This macro is defined in the device-specific code but not on the host. All of our code runs on the host, so this PR uses these wrappers to "reach into" the device code to conditionally define the code.

Results:

If I build

export TORCH_CUDA_ARCH_LIST="7.0 7.5 8.0 8.6 8.9 9.0+PTX"
python setup.py build_ext --inplace

and then run the following (the strip is because I forgot to build in release mode):

strip vllm/_C.cpython-310-x86_64-linux-gnu.so
ls -al vllm/_C.cpython-310-x86_64-linux-gnu.so

main

-rwxrwxr-x 1 tms tms 146108048 Jun  4 20:49 vllm/_C.cpython-310-x86_64-linux-gnu.so

this PR:

-rwxrwxr-x 1 tms tms 142900880 Jun  4 20:04 vllm/_C.cpython-310-x86_64-linux-gnu.so

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[mgoin]

This seems reasonable to me (as in not too bad of a way to achieve this), is this in a land-able state given the passing checks?

[tlrmchlsmth]

is this in a land-able state given the passing checks?

It's in a landable state. I want to measure the final size reduction for the wheel file when compiling for all CUDA ARCHS before marking it ready for review. I started to do this Friday but ran into some very long compile times

edit: was in a landable state -- there will likely be merge conflicts now that #5144 has landed

[tlrmchlsmth]

@mgoin @pcmoritz @comaniac @bnellnm ready for review!