TL:DR Whinge, whinge whinge.
I set up an ML experiment with Optuna using some bike sharing data from UCI. Despite using almost the same code as in previous, successful, experiments, I was getting errors everywhere. Well, I’ve made so many mistakes coding over the years that I’m pretty experienced at finding where the problem is, but in this case something that worked yesterday (on different data) didn’t work today.
I was using a simple model, only one layer with inputs equal to the number of features, and one output (being a regression problem). I tried this on my BTC data a while back and it worked fine. Now, however, the array of predictions was full of null values. If I made the model a little more complex, an extra layer or an activation layer, it worked. But I didn’t need those with the BTC example. So what gives? Usually when I find a problem I understand why it occurred, but not this time. Grr!!