Wrong LaTeX equation in huber_loss_op.cc

[wangkuiyi]

The LaTeX equation of the operator HuberLoss isn't encapsulated by a pair of double-dollar $$:

Paddle/paddle/operators/huber_loss_op.cc

Lines 68 to 79 in c0d2ca5

	Huber loss is a loss function used in robust regression. We define X as the
	input value and Y as the target value. Huber loss can evaluate the fitness of
	X to Y. Different from MSE loss, Huber loss is more robust for outliers. The
	shape of X and Y are [batch_size, 1]. The equation is:
	L_{\delta}(y, f(x)) =
	\begin{cases}
	0.5 * (y - f(x))^2, \quad |y - f(x)| \leq \delta \\
	\delta * (|y - f(x)| - 0.5 * \delta), \quad otherwise
	\end{cases}
	)DOC");

Please add them, while taking the following code as an example:

Paddle/paddle/operators/lrn_op.cc

Lines 89 to 96 in d883547

	The original formula is:
	$$
	Output(i, x, y) = Input(i, x, y) / \left(
	k + \alpha \sum\limits^{\min(C, c + n/2)}_{j = \max(0, c - n/2)}
	(Input(j, x, y))^2
	\right)^{\beta}
	$$