Regression test (pytest) stucks issues found

[hanyong97]

I just ran "python3 -m pytest" in nngen/tests directory.
Found out that follow test cases make the regression test stucks:
"/nngen/tests/onnx_matrix_conv2d/test_onnx_matrix_conv2d_sigmoid_int8_3x3_stride1.py"
"/nngen/tests/onnx_matrix_conv2d/test_onnx_matrix_conv2d_sigmoid_int16_3x3_stride1.py"
"/nngen/tests/onnx_matrix_conv2d/test_onnx_matrix_conv2d_sigmoid_int32_3x3_stride1.py"

From my finding from "/nngen/nngen/quantizer/conv2d.py" found out that the 'rate' always bigger than 'allowed_rate' and stucks in infinity while loop (never execute line: 174) as the increment of 'rshift_out' do not reduce the overflow issues.

As the increament of 'rshift_out', the 'rate' and 'num_overflow' increase as shown:

Please suggest any fixes to solve this issues.

Thanks
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