remove early out in xnnpack pruning policy model traversal

[christian-steinmeyer]

If there are multiple input layers (and therefore paths), this early out might return an empty set before all paths are checked. By simply removing it, the path that doesn't have a next layer ends, nothing is added to the found_layers and the other paths continue to be checked.

I stumbled upon this with a model that had multiple inputs, one of which was feeding directly into the last (custom) layer. As soon as the traversal found no next layer after it, I got the error message of no valid input branch:

'Could not find Conv2D 3x3 layer with stride 2x2, input filters == 3 and VALID padding and preceding ZeroPadding2D with padding == 1 in all input branches of the model'