Source of validation accuracy in zero-cost case

In the zero-cost branch optimizers `Npenas` and `Bananas`, the validation accuracy of architectures is being queried from the zero-cost-benchmark as follows:

```python
model.accuracy = self.zc_api[str(model.arch_hash)]['val_accuracy']
```

The question is, whether this supports the case where the user wants to use the `ZeroCost` predictor because their dataset or search space is not supported by the zero-cost benchmark.

If this is a case that we want to support, one option would be to introduce a parameter `use_zc_api` and use it as follows:

```python
if self.use_zc_api:
model.accuracy = self.zc_api[str(model.arch_hash)]['val_accuracy']
else:
model.accuracy = model.arch.query(
self.performance_metric, self.dataset, dataset_api=self.dataset_api
)
```