PerVariableTransformer: ignore variables with None category

docs/usage.md

@@ -42,7 +42,7 @@ The first method is used by `LearningSolver` to construct a concrete Pyomo model
 
 The second and third methods provide an encoding of the instance, which can be used by the ML models to make predictions. In the knapsack problem, for example, an implementation may decide to provide as instance features the average weights, average prices, number of items and the size of the knapsack. The weight and the price of each individual item could be provided as variable features. See `miplearn/problems/knapsack.py` for a concrete example.
 
-An optional method which can be implemented is `instance.get_variable_category(var, index)`, which returns a category (a string, an integer or any hashable type) for each decision variable. If two variables have the same category, `LearningSolver` will use the same internal ML model to predict the values of both variables. By default, all variables belong to the `"default"` category, and therefore only one ML model is used for all variables.
+An optional method which can be implemented is `instance.get_variable_category(var, index)`, which returns a category (a string, an integer or any hashable type) for each decision variable. If two variables have the same category, `LearningSolver` will use the same internal ML model to predict the values of both variables. By default, all variables belong to the `"default"` category, and therefore only one ML model is used for all variables. If the returned category is `None`, ML predictors will ignore the variable.
 
 It is not necessary to have a one-to-one correspondence between features and problem instances. One important (and deliberate) limitation of MIPLearn, however, is that `get_instance_features()` must always return arrays of same length for all relevant instances of the problem. Similarly, `get_variable_features(var, index)` must also always return arrays of same length for all variables in each category. It is up to the user to decide how to encode variable-length characteristics of the problem into fixed-length vectors. In graph problems, for example, graph embeddings can be used to reduce the (variable-length) lists of nodes and edges into a fixed-length structure that still preserves some properties of the graph. Different instance encodings may have significant impact on performance.
 

miplearn/transformers.py

@@ -57,6 +57,8 @@ class PerVariableTransformer:
         for var in model.component_objects(Var):
             for index in var:
                 category = instance.get_variable_category(var, index)
+                if category is None:
+                    continue
                 if category not in result.keys():
                     result[category] = []
                 result[category] += [(var, index)]