Refer to the original map/zip classes via builtins

Referring to them as simply `map` or `zip` will create recursive
reimplementations (with no base case!) if the cell is reevaluated in
the same runtime.

docs/autodidax.ipynb

@@ -570,6 +570,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "import builtins\n",
+    "\n",
     "def zeros_like(val):\n",
     "  aval = get_aval(val)\n",
     "  return np.zeros(aval.shape, aval.dtype)\n",
@@ -581,17 +583,15 @@
     "    lst2.append(x2)\n",
     "  return lst1, lst2\n",
     "\n",
-    "map_ = map\n",
     "def map(f, *xs):\n",
-    "  return list(map_(f, *xs))\n",
+    "  return list(builtins.map(f, *xs))\n",
     "\n",
-    "zip_ = zip\n",
     "def zip(*args):\n",
     "  fst, *rest = args = map(list, args)\n",
     "  n = len(fst)\n",
     "  for arg in rest:\n",
     "    assert len(arg) == n\n",
-    "  return list(zip_(*args))"
+    "  return list(builtins.zip(*args))"
    ]
   },
   {

docs/autodidax.md

@@ -460,6 +460,8 @@ that now we can add some real transformations.
 First, a few helper functions:
 
 ```{code-cell}
+import builtins
+
 def zeros_like(val):
   aval = get_aval(val)
   return np.zeros(aval.shape, aval.dtype)
@@ -471,17 +473,15 @@ def unzip2(pairs):
     lst2.append(x2)
   return lst1, lst2
 
-map_ = map
 def map(f, *xs):
-  return list(map_(f, *xs))
+  return list(builtins.map(f, *xs))
 
-zip_ = zip
 def zip(*args):
   fst, *rest = args = map(list, args)
   n = len(fst)
   for arg in rest:
     assert len(arg) == n
-  return list(zip_(*args))
+  return list(builtins.zip(*args))
 ```
 
 The `Tracer` for forward-mode autodiff carries a primal-tangent pair. The

docs/autodidax.py

@@ -444,8 +444,6 @@ def f(x):
   return z
 
 print(f(3.0))
-
-
 # -
 
 # Woo! Like going around in a big circle. But the point of this indirection is
@@ -456,6 +454,8 @@ print(f(3.0))
 # First, a few helper functions:
 
 # +
+import builtins
+
 def zeros_like(val):
   aval = get_aval(val)
   return np.zeros(aval.shape, aval.dtype)
@@ -467,17 +467,15 @@ def unzip2(pairs):
     lst2.append(x2)
   return lst1, lst2
 
-map_ = map
 def map(f, *xs):
-  return list(map_(f, *xs))
+  return list(builtins.map(f, *xs))
 
-zip_ = zip
 def zip(*args):
   fst, *rest = args = map(list, args)
   n = len(fst)
   for arg in rest:
     assert len(arg) == n
-  return list(zip_(*args))
+  return list(builtins.zip(*args))
 
 
 # -