Replace duplicative s with alpha (pytorch#56804)

Summary:
It is always easier to read a document when different objects / concepts denoted with different variables / representations.
In this PR we make sure the [complex autograd](https://pytorch.org/docs/master/notes/autograd.html#autograd-for-complex-numbers) documentation, the variable of output and step size diverge.

Fixes pytorch#53633

Pull Request resolved: pytorch#56804

Reviewed By: anjali411

Differential Revision: D27989959

Pulled By: iramazanli

fbshipit-source-id: c271590ee744c8aeeff62bfaa2295429765ef64e

Author: iramazanli

docs/source/notes/autograd.rst

@@ -330,22 +330,22 @@ How is Wirtinger Calculus useful in optimization?
 Researchers in audio and other fields, more commonly, use gradient
 descent to optimize real valued loss functions with complex variables.
 Typically, these people treat the real and imaginary values as separate
-channels that can be updated. For a step size :math:`s/2` and loss
+channels that can be updated. For a step size :math:`\alpha/2` and loss
 :math:`L`, we can write the following equations in :math:`ℝ^2`:
 
     .. math::
         \begin{aligned}
-            x_{n+1} &= x_n - (s/2) * \frac{\partial L}{\partial x}  \\
-            y_{n+1} &= y_n - (s/2) * \frac{\partial L}{\partial y}
+            x_{n+1} &= x_n - (\alpha/2) * \frac{\partial L}{\partial x}  \\
+            y_{n+1} &= y_n - (\alpha/2) * \frac{\partial L}{\partial y}
         \end{aligned}
 
 How do these equations translate into complex space :math:`ℂ`?
 
     .. math::
         \begin{aligned}
-            z_{n+1} &= x_n - (s/2) * \frac{\partial L}{\partial x} + 1j * (y_n - (s/2) * \frac{\partial L}{\partial y}) \\
-                    &= z_n - s * 1/2 * (\frac{\partial L}{\partial x} + j \frac{\partial L}{\partial y}) \\
-                    &= z_n - s * \frac{\partial L}{\partial z^*}
+            z_{n+1} &= x_n - (\alpha/2) * \frac{\partial L}{\partial x} + 1j * (y_n - (\alpha/2) * \frac{\partial L}{\partial y}) \\
+                    &= z_n - \alpha * 1/2 * (\frac{\partial L}{\partial x} + j \frac{\partial L}{\partial y}) \\
+                    &= z_n - \alpha * \frac{\partial L}{\partial z^*}
         \end{aligned}
 
 Something very interesting has happened: Wirtinger calculus tells us