I figured out what is happened.
FNN is started working after changing:
target_train = autograd.Variable(y_train_tens)
to
target_train = y_train_tens.squeeze(1)
I calculate a confusion matrix (using “sklearn  metrics”).
However, I got always same result for 4 class classification:
[[530783      0      0      0]
[  8097      0      0      0]
[ 20079      0      0      0]
[ 16682      0      0      0]]
Where can be an error?