Fix/trace mode

[sebffischer]

Bugfixes:

• save-load roundtrip used to convert script_function into script_module. This is fixed now (in a somewhat hacky way)

Open questions.

• In this PR, we only do the train-eval distinction for the $forward method. Maybe we want to do this for all methods.
• Can we register the $forward method (that distinguishes between the internal train and eval forward method) in the underlying script module instead of the nn_Module?
• Reading a script function is currently loaded as a module, I don't think this should be the case

TODOs:

• the training mode after saving a script_module is changed. This should probably be fixed.
• Currently, after saving a trace-jitted module, the $forward method is not defined. This is because the underlying module class is not saved when saving a trace jitted module, so when loading it again, the unpatched methods are used.
• Properly distinguish between saved module and saved function. The current hack does not work for jitted modules that were saved with an old version.

[sebffischer]

Superseded by: #1217