Raft refacto

[jsalotti]

Make code more generic and readable :

• the class of correlation block can be given as a parameter of the constructor
• flow upsampling logic is factorized in method upsample_flow()
• the outputs of an update step are regrouped in a dict ;
• at each update step, the output dict is appended to list m_outputs
• the processing after last update step is factorized in forward_head
• out_plane allows to predict a map of vectors of different sizes (1D, 2D, 3D, etc.)
• added properties hdim and cdim as short_names for hidden_dim and context_dim

Better implementation for abstract class attributes :

• declare abstract attribute in parent class with my_attribute = abstract_attribute()
• check that abstract attributes are instantiated in child class by overriding parent class __new__ method
• two methods are provided to easily override __new__ : check_abstract_attribute_instanciation and super_new

[thibo73800]

@jsalotti can you provide a description of the changes ?