convert-hf : support direct Q8_0 conversion (#7234)

* convert-hf : support q8_0 conversion

* convert-hf : add missing ftype

This was messing with the checksums otherwise.

* convert-hf : add missing ftype to Baichuan and Xverse

I didn't notice these on my first pass.

gguf-py/gguf/__init__.py

@@ -2,5 +2,6 @@ from .constants import *
 from .lazy import *
 from .gguf_reader import *
 from .gguf_writer import *
+from .quants import *
 from .tensor_mapping import *
 from .vocab import *