[Task]: Add unit tests for `Engine.export`

[ashwinvaidya17]

What is the motivation for this task?

Currently, export types like NNCF and POT do not fall under testing. This leads to outdated docstrings and broken functionality.

Describe the solution you'd like

The test can be as simple as optimizing a pre-trained Padim model. We already have the trained checkpoint available in a fixture.

Additional context

No response