chapter 17 Model.bwd fix (#270)

Remove self. from function parameter which was not member variable so code runs.

17_foundations.ipynb

@@ -2198,7 +2198,7 @@
     "    \n",
     "    def bwd(self, out, inp):\n",
     "        inp.g = out.g @ self.w.t()\n",
-    "        self.w.g = self.inp.t() @ self.out.g\n",
+    "        self.w.g = inp.t() @ self.out.g\n",
     "        self.b.g = out.g.sum(0)"
    ]
   },