In general, it is definetely very computationally expensive, so the exhaustive search is not perfromed in pratice, however, there are some recent approaches for determining, whether the architecture is fine, without performing the training - by looking at the covariance matrix after forwarding the data, for example, in a recent paper - https://arxiv.org/abs/2006.04647. However such approach is limited by far.

In general, it is definitely very computationally expensive, so an exhaustive search is not performed in practice. However, there are some recent approaches for determining whether the architecture is "fine" without training the neural network first - by looking at the covariance matrix after forwarding the data, for example, in a recent paper Neural Architecture Search without Training. However, such an approach is very limited.