Minor improvements in the module

[adekusar-drl]

What is the expected enhancement?

There are minor improvements to do:

• algorithms/init

• NNC/NNR/VQR are missing in docstring

• datasets

• Return np.ndarrays, optionally one-hot encoded (remove all functions not used)

• neural_networks/init

• OpflowQNN missing in docstring

• loss_functions

• Should we rename the file loss.py to loss_functions.py?

• Should we include the actual loss functions in the init.py to make them importable directly, e.g. "from qml.utils.lossfunctions import CrossEntropyLoss"? (currently there is only the copyright header in the loss_functions init)

• Are we using L2LossProbability or should we remove it? Or is it tested somewhere?

• We should extend the docstrings of the loss functions a little bit to include what exactly they are computing.

• Did we test the KLDiv somewhere (it doesn't have a gradient implemented)?

• Do we need softmax/stable_softmax, doesn't seem to be used anywhere, maybe remove?

[adekusar-drl]

Datasets are fixed in #26