add method to set default kind of new variables in VarStore

[jquesnelle]

This changes the behavior of VarStore::set_kind to change the default kind for all new variables created in the VarStore, similar to torch.set_default_dtype (but localized to the VarStore).

The need for this is that for larger models like Llama-7b first instantiating an empty model and then filling in the tensors from an mmap-ed safetenstors file (for example) won't work on a 24GB GPU since a 7B won't fit in Kind::Float. Rather, newly inited tensors need to start out as Kind::Half or Kind::BFloat16.

The default behavior of VarStore is unchanged -- existing code that doesn't call set_kind will have new tensors created at as Kind::Float.

[LaurentMazare]

Thanks!