Add math explanation

Author: Atcold

Deep Learning with PyTorch.ipynb

@@ -420,6 +420,15 @@
     "x.grad"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "You should have got a matrix of `4.5`.\n",
+    "Let's call the `out` *Variable* \"$o$\".  \n",
+    "We have that $o = \\frac{1}{4}\\sum_i z_i$, $z_i = 3(x_i+2)^2$ and $z_i\\bigr\\rvert_{x_i=1} = 27$. Therefore, $\\frac{\\partial o}{\\partial x_i} = \\frac{3}{2}(x_i+2)$, hence $\\frac{\\partial o}{\\partial x_i}\\bigr\\rvert_{x_i=1} = \\frac{9}{2} = 4.5$."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},