Add support for minifloat ptq with fx backend on residual models

[fabianandresgrob]

When we quantize a model using a minifloat format and the fx backend, the residual handler tries to access QuantAct modules' int_scaling_impl, as seen here.

This throws an error as minifloat configurations will have a FloatQuant quantizer and therefore no int_scaling_impl but a float_scaling_impl.
This only occurs with the fx backend.