Move formulas on separate line in loss.py (pytorch#150565)

Move formulas on separate line in loss.py for better readability.

Pull Request resolved: pytorch#150565
Approved by: https://github.com/mikaylagawarecki

Author: svekars

torch/nn/modules/loss.py

@@ -154,8 +154,8 @@ class NLLLoss(_WeightedLoss):
     The unreduced (i.e. with :attr:`reduction` set to ``'none'``) loss can be described as:
 
     .. math::
-        \ell(x, y) = L = \{l_1,\dots,l_N\}^\top, \quad
-        l_n = - w_{y_n} x_{n,y_n}, \quad
+        \ell(x, y) = L = \{l_1,\dots,l_N\}^\top, \\
+        l_n = - w_{y_n} x_{n,y_n}, \\
         w_{c} = \text{weight}[c] \cdot \mathbb{1}\{c \not= \text{ignore\_index}\},
 
     where :math:`x` is the input, :math:`y` is the target, :math:`w` is the weight, and