refactor onnxslim test and fix bugs

[whyb]

Closes #42

Added OnnxSlim command test.

• Test the basic functionality of the slim function
• Test the modification of input shapes.
• Test the conversion of an ONNX model from FP32 to FP16 precision.
• Test the conversion of an ONNX model from FP16 to FP32 precision.

[inisis]

Good Job!

[whyb]

@inisis

Could not merge this pr to main branch, Because CI test failed.

Test command:

onnxslim --input-shapes input:1,3,224,224 "/data/modelzoo/resnet18/resnet18.onnx" "/tmp/tmpj1kh/resnet18_fp16.onnx"

Failed message:

 =========================== short test summary info ============================
FAILED tests/test_onnxslim.py::TestFunctional::test_input_shape_modification - assert 2 == 0
 +  where 2 = CompletedProcess(args='onnxslim --input-shapes input:1,3,224,224 "/data/modelzoo/resnet18/resnet18.onnx" "/tmp/tmpj1kh...-v]\n                input_model [output_model]\nonnxslim: error: the following arguments are required: input_model\n').returncode
FAILED tests/test_onnxslim.py::TestFunctional::test_fp322fp16_conversion - FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpgmnl41ah/resnet18_fp16.onnx'
================== 2 failed, 5 passed, 107 warnings in 10.90s ==================

[inisis]

@whyb Thanks for your contributions!