mnist: support --model-path and --load-model

[Rtoax]

If specify --load-model will only run test(), if specify --model-path=A, the saved model will be renamed to A.

Example:

python main.py --save-model --model-path=mnist_cnn.pt
python main.py --load-model=mnist_cnn.pt

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[Rtoax]

@smartkiwi Could you review this PR? thanks a lot.