update docstring or fix constraints in nir_graph_extractor.py

[ssinhaleite]

Error message on nir_graph_extrator:

if self.dvs_layer.merge_polarities and features != 1:
            raise ValueError(
                f"The 'DVSLayer.merge_polarities' is set to 'True' which means the number of input features should be 1 (current input shape is {dvs_input_shape})."

Evaluate this error and method. It seems a harder constraint is being imposed than the docstring explains.
(Two is the correct option if merge_polarities is False.)