Add torch API usage example

[timonmerk]

Addresses #97

Added minimal example of torch API.

In comparison to the get_emissions function in the https://cebra.ai/docs/demo_notebooks/Demo_Allen.html notebook, I used the solver.transform function, to keep the example a bit shorter

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[timonmerk]

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[stes]

Thanks @timonmerk for the contribution! I left some small comments to adapt the style of the example to the remainder of the code base and also started the test suite (the code in the usage.rst is tested as well).

[stes]

lgtm now, requesting review also from @MMathisLab

[MMathisLab]

Looks good! I'd make Cebra ~> CEBRA and need to check why the tests fail :)

[stes]

actually, just noticed a few additional minor edits re the use of the __len__ functions... will edit

[stes]

added a few comments to improve the example. apologies for missing these earlier. @timonmerk , no action needed here, I can do the edits

[MMathisLab]

@stes okay to merge, and can be updated based on updates from @gonlairo later

[MMathisLab]

@stes I think this should be merged, and if you have improvements and input from @gonlairo we can add them, but nothing seems technically wrong, and it's been open >30 days, so I'd like to merge.

can be addressed in another PR