Hi,

I know that this error implies that the computational graph has been used previously, and one should not use it twice.

No it is not what this error means. This error means that a Tensor that is needed to compute the backward has been modified inplace. And so the backward cannot be computed anymore.

However - what does “[torch.FloatTensor [1]]” specifiy? And what does “version 2” and “version 1” mean?

This means that the faulty Tensor (that was modified inplace) is a float Tensor of shape [1] and is at version 2 (second time it has been modified inplace) while the autograd expect version 1 (the result after it was modified inplace once).