gguf-py : simplify support for quant types (#8838)

* gguf-py : use classes for quants

* convert_hf : simplify internal quantization type selection

* gguf-py : fix flake8 lint

* gguf-py : fix BF16 numpy view type

* gguf-py : remove LlamaFileTypeMap

Too specific to 'llama.cpp', and would be a maintenance burden
to keep up to date.

* gguf-py : add generic quantize and dequantize functions

The quant classes no longer need to be known,
only the target or the source type,
for 'quantize' and 'dequantize', respectively.

gguf-py/gguf/lazy.py

@@ -191,6 +191,8 @@ class LazyBase(ABC, metaclass=LazyMeta):
 class LazyNumpyTensor(LazyBase):
     _tensor_type = np.ndarray
 
+    shape: tuple[int, ...]  # Makes the type checker happy in quants.py
+
     @classmethod
     def meta_with_dtype_and_shape(cls, dtype: DTypeLike, shape: tuple[int, ...]) -> np.ndarray[Any, Any]:
         # The initial idea was to use np.nan as the fill value,