Refresh some of our Colab notebooks. (#6162)

* Change license header style to collapsed sections (see discussion: https://github.com/google/iree/pull/5990#discussion_r643414147)
* Restore original copyright years (errant find/replace set all to 2020)
* Run notebooks
  * Update for errors (e.g. swap `mhlo` op -> `std` op in low_level_invoke_function)
  * Switch resnet from vmvx to llvmaot (vmvx was way too slow)

---

I did not update these notebooks:

* mnist_training.ipynb (is currently broken with a crash in `iree-import-tf`: https://github.com/google/iree/issues/6163)
* tflite_text_classification.ipynb (is currently broken from an importer change, `tosa` ops exist and no `-iree-input-type=` flag is set)
diff --git a/colab/tensorflow_hub_import.ipynb b/colab/tensorflow_hub_import.ipynb
index a4d7209..9a5b7b3 100644
--- a/colab/tensorflow_hub_import.ipynb
+++ b/colab/tensorflow_hub_import.ipynb
@@ -6,7 +6,8 @@
       "name": "tensorflow_hub_import.ipynb",
       "provenance": [],
       "collapsed_sections": [
-        "-V0X0E7LkEa4"
+        "-V0X0E7LkEa4",
+        "FH3IRpYTta2v"
       ]
     },
     "kernelspec": {
@@ -21,19 +22,27 @@
     {
       "cell_type": "markdown",
       "metadata": {
-        "id": "license"
+        "id": "FH3IRpYTta2v"
       },
       "source": [
-        "```\n",
-        "Copyright 2020 The IREE Authors\n",
-        "\n",
-        "Licensed under the Apache License v2.0 with LLVM Exceptions.\n",
-        "See https://llvm.org/LICENSE.txt for license information.\n",
-        "SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception\n",
-        "```\n"
+        "##### Copyright 2021 The IREE Authors"
       ]
     },
     {
+      "cell_type": "code",
+      "metadata": {
+        "id": "mWGa71_Ct2ug",
+        "cellView": "form"
+      },
+      "source": [
+        "#@title Licensed under the Apache License v2.0 with LLVM Exceptions.\n",
+        "# See https://llvm.org/LICENSE.txt for license information.\n",
+        "# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception"
+      ],
+      "execution_count": 1,
+      "outputs": []
+    },
+    {
       "cell_type": "markdown",
       "metadata": {
         "id": "Qb3S0mSjpK7J"
@@ -70,7 +79,7 @@
         "%%capture\n",
         "!python -m pip install iree-compiler-snapshot iree-runtime-snapshot iree-tools-tf-snapshot -f https://github.com/google/iree/releases"
       ],
-      "execution_count": 1,
+      "execution_count": 2,
       "outputs": []
     },
     {
@@ -80,7 +89,7 @@
         "colab": {
           "base_uri": "https://localhost:8080/"
         },
-        "outputId": "15da7042-578c-4cc9-8c54-e1e22d4fc6f1"
+        "outputId": "31418cc7-ecd5-4c73-c595-ff1d6caf03cc"
       },
       "source": [
         "import os\n",
@@ -98,7 +107,7 @@
         "os.makedirs(ARTIFACTS_DIR, exist_ok=True)\n",
         "print(f\"Using artifacts directory '{ARTIFACTS_DIR}'\")"
       ],
-      "execution_count": 2,
+      "execution_count": 3,
       "outputs": [
         {
           "output_type": "stream",
@@ -130,7 +139,7 @@
           "base_uri": "https://localhost:8080/"
         },
         "id": "7fd0vmnloZo9",
-        "outputId": "71ac2d17-d7ea-4982-c3d6-f709e394a37e"
+        "outputId": "fcb260e8-672a-479d-c428-2a93d306d4b2"
       },
       "source": [
         "#@title Download the pretrained model\n",
@@ -141,7 +150,7 @@
         "model_path = hub.resolve(HUB_PATH)\n",
         "print(f\"Downloaded model from tfhub to path: '{model_path}'\")"
       ],
-      "execution_count": 3,
+      "execution_count": 4,
       "outputs": [
         {
           "output_type": "stream",
@@ -175,7 +184,7 @@
           "base_uri": "https://localhost:8080/"
         },
         "id": "qiO66oEYQmsd",
-        "outputId": "23dabcab-5e97-488f-a133-7614666f6609"
+        "outputId": "5d7e4f7d-e77e-45f1-c37e-11e38ed05ce2"
       },
       "source": [
         "#@title Check for serving signatures\n",
@@ -192,7 +201,7 @@
         "print(\"Checking for signature_defs using saved_model_cli:\\n\")\n",
         "!saved_model_cli show --dir {model_path} --tag_set serve --signature_def serving_default"
       ],
-      "execution_count": 4,
+      "execution_count": 5,
       "outputs": [
         {
           "output_type": "stream",
@@ -230,7 +239,7 @@
           "base_uri": "https://localhost:8080/"
         },
         "id": "OlDG2OuqOBGC",
-        "outputId": "0e3be224-a373-441f-cb8f-aaf2b19e0d26"
+        "outputId": "044bb741-a563-4fe2-b0d9-8b47bcc5f286"
       },
       "source": [
         "#@title Look up input signatures to use when exporting\n",
@@ -242,7 +251,7 @@
         "!saved_model_cli show --dir {model_path} --all \\\n",
         "    2> /dev/null | grep \"inputs: TensorSpec\" | tail -n 1"
       ],
-      "execution_count": 5,
+      "execution_count": 6,
       "outputs": [
         {
           "output_type": "stream",
@@ -260,7 +269,7 @@
           "base_uri": "https://localhost:8080/"
         },
         "id": "gnb4HhMmkgiT",
-        "outputId": "9a223ee5-cc52-4580-be88-5073a75dd97a"
+        "outputId": "3318bc8e-321c-417f-d9b0-b4cc9dcca8de"
       },
       "source": [
         "#@title Re-export the model using the known signature\n",
@@ -290,7 +299,7 @@
         "print(\"Checking for signature_defs using saved_model_cli:\\n\")\n",
         "!saved_model_cli show --dir {resaved_model_path} --tag_set serve --signature_def serving_default"
       ],
-      "execution_count": 6,
+      "execution_count": 7,
       "outputs": [
         {
           "output_type": "stream",
@@ -336,7 +345,7 @@
           "base_uri": "https://localhost:8080/"
         },
         "id": "GLkjlHE5mdmg",
-        "outputId": "d174d83d-350e-490e-f5e8-d4419f0c633a"
+        "outputId": "d057d2dc-001c-4fe4-cf4b-9517d4cf598e"
       },
       "source": [
         "#@title Import from SavedModel\n",
@@ -370,7 +379,7 @@
         "print(f\"Saved tf_input to        '{tf_input}'\")\n",
         "print(f\"Saved iree_input to      '{iree_input}'\")"
       ],
-      "execution_count": 7,
+      "execution_count": 8,
       "outputs": [
         {
           "output_type": "stream",
@@ -391,7 +400,7 @@
           "height": 102
         },
         "id": "IEJAzOb5qASI",
-        "outputId": "3515b4ef-fd6a-4f14-ecd1-0676e246a089"
+        "outputId": "52da0e1f-a512-4ee8-ea65-e716de9a3cf2"
       },
       "source": [
         "#@title Download compilation artifacts\n",
@@ -406,14 +415,14 @@
         "print(\"Downloading the artifacts zip file...\")\n",
         "files.download(ARTIFACTS_ZIP)"
       ],
-      "execution_count": 8,
+      "execution_count": 9,
       "outputs": [
         {
           "output_type": "stream",
           "text": [
             "Zipping '/tmp/iree/colab_artifacts' to '/tmp/mobilenet_colab_artifacts.zip' for download...\n",
-            "  adding: mobilenet_v2_iree_input.mlir (deflated 47%)\n",
             "  adding: mobilenet_v2.vmfb (deflated 8%)\n",
+            "  adding: mobilenet_v2_iree_input.mlir (deflated 46%)\n",
             "  adding: mobilenet_v2_tf_input.mlir (deflated 47%)\n",
             "Downloading the artifacts zip file...\n"
           ],
@@ -477,7 +486,7 @@
           "output_type": "display_data",
           "data": {
             "application/javascript": [
-              "download(\"download_64f7173f-1d8f-42f2-b184-0c2feb0700f4\", \"mobilenet_colab_artifacts.zip\", 43803643)"
+              "download(\"download_173f97e9-1e2d-401a-8aad-646463432451\", \"mobilenet_colab_artifacts.zip\", 59101356)"
             ],
             "text/plain": [
               "<IPython.core.display.Javascript object>"