added distiller as a submodule
updated env.yaml
auto-install distiller in installer

Added nvdla/sw as a submodule;
Relaxed python version req to be >= 3.6;
Only install torch2hpvm
Don't download DNN params

When the GCC used to compile tensor runtime is not the system's default,
path to that GCC's GLIBC leaks into hpvm-clang
that can override system's default and cause GLIBC failure.