Implement the hyperbolic tangent (tanh) activation function.
$$\tanh(x) = \frac{e^x - e^{-x}}{e^x + e^{-x}}$$
Equivalently: $\tanh(x) = 2\sigma(2x) - 1$ where $\sigma$ is the sigmoid function.
Input: A tensor x of any shape
Output: A tensor of the same shape with values in $(-1, 1)$