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