Updates mnist_tensorflow to use the vmla driver when running on CPU.

Changes the `driver_name` variable from `"interpreter"` to `"vmla"` so that the notebook works when not using Vulkan.

I also added `**/.ipynb_checkpoints/` to `.gitignore` to prevent that from accidentally being added.

Closes https://github.com/google/iree/pull/1338

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/1338 from google:phoenix-meadowlark-patch-1 bb461324d5a5a1d2a9c9d9af085c46cbb4bf4405
PiperOrigin-RevId: 304209468

.gitignore

diff --git a/.gitignore b/.gitignore
index 990e1ad..c8b7550 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@
 
 # Python
 *.pyc
+**/.ipynb_checkpoints/
 
 # Visual Studio files
 .vs/

colab/mnist_tensorflow.ipynb

diff --git a/colab/mnist_tensorflow.ipynb b/colab/mnist_tensorflow.ipynb
index 590f36e..229bab3 100644
--- a/colab/mnist_tensorflow.ipynb
+++ b/colab/mnist_tensorflow.ipynb
@@ -134,7 +134,7 @@
         "  driver_name = \"vulkan\"\n",
         "else:\n",
         "  backend_name = \"vmla\"\n",
-        "  driver_name = \"interpreter\"\n",
+        "  driver_name = \"vmla\"\n",
         "tf.print(\"Using IREE compiler backend '%s' and runtime driver '%s'\" % (backend_name, driver_name))\n",
         "\n",
         "#@markdown -----\n",
@@ -623,4 +623,4 @@
       ]
     }
   ]
-}
\ No newline at end of file
+}