fix(cpu): correctly handle 1D output shape in dequantize_4bit

[SparshM8]

Description

Fixes #2047.

The CPU kernel for dequantize_4bit was unconditionally unsqueezing 1D shapes to (1, n) to satisfy C-library pointer requirements, but it failed to preserve the original 1D shape in the returned tensor. This resulted in an unexpected (1, N) output for a requested (N,) shape, which could cause dimension mismatch errors in downstream code.

Changes

• Modified bitsandbytes/backends/cpu/ops.py to use an out_ptr (an unsqueezed view) for C-library calls while maintaining the original out tensor's shape.
• Applied the same out_ptr pattern to quantize_blockwise, dequantize_blockwise, and gemv_4bit kernels for consistency and safety.
• Added a new regression test tests/test_issue_2047.py that verifies 1D output shapes for all supported dtypes and quantization types on available devices.

Testing

Verified with the new regression test on CPU:

PYTHONPATH=. pytest tests/test_issue_2047.py

Output: 12 passed in 18.96s

[caiotheodoro]

#2048 fixes the same root cause in 4 lines: shape gets reassigned to (1, shape[0]) and that mutated value is what torch.empty(shape, ...) uses for out, but m/n don't need the reassignment (prod(()) and prod((1,)) are both 1). Deleting the reassignment fixes it without touching anything else.

This PR's out_ptr change also touches quantize_blockwise and dequantize_blockwise (lines 43 and 97), which don't have the shape bug at all — out_ptr = out there is a no-op alias, pure diff noise on unrelated functions. Worth closing this in favor of #2048, or at minimum scoping it down to just dequantize_4bit and dropping the rename of the registered kernel from _ to dequantize_4bit_cpu (every other kernel in this file keeps the anonymous _ convention).