[Help wanted] Support BUILD_FOR_ALL_ARCHS

[csukuangfj]

To prevent the following error,

we use

option(K2_BUILD_FOR_ALL_ARCHS "Whether to build k2 for all GPU architectures" OFF)

https://github.com/k2-fsa/k2/blob/efd83642a940dc7db08688cc0791985bed1fafcd/CMakeLists.txt#L87

in k2.

It would be nice if someone can port K2_BUILD_FOR_ALL_ARCHS to this reposiotry.