How would you like to define your accuracy?
If you are dealing with a multi-class classification use case, you could compare the predictions to the targets via:
output = torch.randn(10, 10)
target = torch.randint(0, 10, (10,))
preds = torch.argmax(output, 1)
acc = preds.eq(target).float().mean()

Usually you wouldn’t use nn.MSELoss for a classification use case, so we would need some information about your use case.