Update lr_from_k_sq Function to Handle k_sq → 0 Limit for Exponents > 3

[E-Rum]

In #128, we decided to add a condition for background correction for now, without updating the version of the lr_from_k_sq function. Currently, this function returns 0 for every exponent when k_sq = 0. However, this behavior is incorrect since the function is actually defined for exponents > 3. Therefore, we need to update this function accordingly.

It is important to note that these values may only be defined in the limit as k_sq → 0. Thus, we must ensure this is handled correctly in Torch.