docs: Update multimodal notebook to use public runtime helpers

Author: shuoweil

notebooks/multimodal/multimodal_dataframe.ipynb

@@ -134,7 +134,7 @@
       "metadata": {},
       "outputs": [],
       "source": [
-        "import bigframes.operations as ops\n",
+        "import bigframes.bigquery as bbq\n",
         "\n",
         "def get_runtime_json_str(series, mode=\"R\", with_metadata=False):\n",
         "    \"\"\"\n",
@@ -148,16 +148,16 @@
         "    \"\"\"\n",
         "    # 1. Optionally fetch metadata\n",
         "    s = (\n",
-        "        series._apply_unary_op(ops.obj_fetch_metadata_op)\n",
+        "        bbq.obj.fetch_metadata(series)\n",
         "        if with_metadata\n",
         "        else series\n",
         "    )\n",
         "    \n",
         "    # 2. Retrieve the access URL runtime object\n",
-        "    runtime = s._apply_unary_op(ops.ObjGetAccessUrl(mode=mode))\n",
+        "    runtime = bbq.obj.get_access_url(s, mode=mode)\n",
         "    \n",
         "    # 3. Convert the runtime object to a JSON string\n",
-        "    return runtime._apply_unary_op(ops.ToJSONString())"
+        "    return bbq.to_json_string(runtime)"
       ]
     },
     {
@@ -1536,7 +1536,6 @@
       ],
       "source": [
         "import bigframes.bigquery as bbq\n",
-        "import bigframes.operations as ops\n",
         "\n",
         "# The audio_transcribe function is a convenience wrapper around bigframes.bigquery.ai.generate.\n",
         "# Here's how to perform the same operation directly:\n",
@@ -1551,8 +1550,8 @@
         "\n",
         "# Convert the audio series to the runtime representation required by the model.\n",
         "# This involves fetching metadata and getting a signed access URL.\n",
-        "audio_metadata = audio_series._apply_unary_op(ops.obj_fetch_metadata_op)\n",
-        "audio_runtime = audio_metadata._apply_unary_op(ops.ObjGetAccessUrl(mode=\"R\"))\n",
+        "audio_metadata = bbq.obj.fetch_metadata(audio_series)\n",
+        "audio_runtime = bbq.obj.get_access_url(audio_metadata, mode=\"R\")\n",
         "\n",
         "transcribed_results = bbq.ai.generate(\n",
         "    prompt=(prompt_text, audio_runtime),\n",