Unpredictable quantization with quantized_bits and an alpha value of None

https://github.com/google/qkeras/blob/dc0bce96c269750ca21f8fc0c44864011d7cdc43/qkeras/quantizers.py#L1287 states that leaving `alpha` at the default value of `None` should result in `alpha` being set to `1`, but that's not the case which leads to unexpected behavior in quantization. The result is quantized values that are out of range a lot.

Manually setting alpha to 1 in the constructor leads to the expected results after quantization