Negative dimensions start from the end, so -1 would be the last dimension, -2 the one before etc.:
        [1, 256, 7, 7]
pos dim: 0,   1,  2, 3
neg dim: -4, -3, -2, -1

If you are passing dimensions outside of this range, you should get an error:
IndexError: Dimension out of range (expected to be in range of [-4, 3], but got -5)

dim=3 would correspond to dim=-1.