'Associates `op_name` key with `pxg_class` value.
Registers `pxg_class` as the class that will be called to perform
per-example differentiation through ops with `op_name`.
Args:
op_name: String op name.
pxg_class: An instance of any class with the same signature as MatMulPXG.'

def Register(self, op_name, pxg_class):

    self.d[op_name] = pxg_class
