gguf-py : fix and simplify quantized shape round-trip (#7483)

* gguf-py : fix and simplify quantized shape round-trip

* gguf-py : remove unused import

gguf-py/gguf/quants.py

@@ -1,5 +1,5 @@
 from __future__ import annotations
-from typing import Callable
+from typing import Callable, Sequence
 
 from numpy.typing import DTypeLike
 
@@ -9,6 +9,20 @@ from .lazy import LazyNumpyTensor
 import numpy as np
 
 
+def quant_shape_to_byte_shape(shape: Sequence[int], quant_type: GGMLQuantizationType):
+    block_size, type_size = GGML_QUANT_SIZES[quant_type]
+    if shape[-1] % block_size != 0:
+        raise ValueError(f"Quantized tensor row size ({shape[-1]}) is not a multiple of {quant_type.name} block size ({block_size})")
+    return (*shape[:-1], shape[-1] // block_size * type_size)
+
+
+def quant_shape_from_byte_shape(shape: Sequence[int], quant_type: GGMLQuantizationType):
+    block_size, type_size = GGML_QUANT_SIZES[quant_type]
+    if shape[-1] % type_size != 0:
+        raise ValueError(f"Quantized tensor bytes per row ({shape[-1]}) is not a multiple of {quant_type.name} type size ({type_size})")
+    return (*shape[:-1], shape[-1] // type_size * block_size)
+
+
 # same as ggml_compute_fp32_to_bf16 in ggml-impl.h
 def __compute_fp32_to_bf16(n: np.ndarray) -> np.ndarray:
     n = n.astype(np.float32, copy=False).view(np.int32)