Minor fix in SGDOptimizer.learning_rates_setup

only a minor fix in 
SGDOptimizer.learning_rates_setup()

I was getting a warning
```bash
"Warning: in SGDOptimizer, keyword argument w_z_in_lr will be ignored, because no parameter was found with name w_z_in." 
```
although the provided parameter list contained a shared variable with name 'w_z_in' and self.learning_rates was updated accordingly (set to the specified value, which was not the default 1.0).