Ok, thanks for trying!
It seems you are missing the last conv layer in your forward pass, but I assume it’s a typo like the missing return statement.
In summary: your model does not learn anyhing, neither the training sequences nor the validation/test sequences.
The losses stay the same and the model’s output is all zeros.
In this case, I would try to boil down the problem by making the problem simpler.
One approach to see if your model architecture is good at all is to use very few samples (you could also use just one sample) and to try to overfit badly on it.
If this doesn’t work at all, we can dig a bit deeper.
Could you try that and report your results?