Fix sign flip during quantization

[TamimEhsan]

The 1 bit code is quantized comparing positive vs negative or zero

RaBitQ-Library/include/rabitqlib/quantization/rabitq_impl.hpp

Line 53 in b4d4e97

x_u = (residual_arr > 0).template cast<int>();

However, during ex bit quantization, the signs are flipped under condition negative vs positive or zero.

RaBitQ-Library/include/rabitqlib/quantization/rabitq_impl.hpp

Lines 437 to 444 in b4d4e97

	// revert codes for negative dims
	int32_t mask = (1 << ex_bits) - 1;
	for (size_t j = 0; j < dim; ++j) {
	if (res_arr.data()[j] < 0) {
	TP tmp = ex_code[j];
	ex_code[j] = (~tmp) & mask;
	}
	}

There's no change in the recall on the indices.