added imports to semi-supervised pre-training ex

[tylerjthomas9]

The example for semi-supervised pre-training was missing the import for TabNetPretrainer and TabNetClassifier (this one is shown in a previous example, so it might not be needed).

What kind of change does this PR introduce?

Updated docs

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

Nothing

Closing issues

N/a

[eduardocarvp]

Might as well add import torch in this case to keep a minimal fully working version ;)