Clarify that we expect models without softmax activation at the end

[cofri]

Some methods can raise an error which is difficult to understand when a model with softmax is used. For example, VIM tries to get weights of the last layer. If last layer is Softmax, an error is raised:

oodeel/oodeel/methods/vim.py

Line 102 in d07f64c

W, b = self.feature_extractor.get_weights(-1)

Moreover, other methods (e.g. VIM, Energy) do not throw an error if softmax is present. But the performance are very poor and not as expected.

Suggestions:

• clarify documentation/docstrings/etc. to force users to end their models with logits
• OR ensure that models used do not end with activation (error assertion)
• OR automatically remove last activation of model when fit().