Update `mnist_torch.ipynb`

[ma7dev]

Describe the bug
Update example as we have replaced approx.auto_select_backend() with approx.auto_set_backend().

In addition, we will need to add more documentation and add !pip install approxlib at the beginning of the example.

[bushshrub]

%pip install approxlib

[bushshrub]

I somewhat updated this but we still need to take out the Conv2d and whatnot