Enable testing of quimb-dependent notebook (quantumlib#6461)

* Enable Contract-a-Grid-Circuit.ipynb in notebook tests,
  but test it only with the pre-release Cirq.

The notebook requires a pinned version of quimb from quantumlib#6438 otherwise the
tensor_expectation_value() call in the last-cell causes out-of-memory error.

Fixes quantumlib#6088

cirq/contrib/quimb/Contract-a-Grid-Circuit.ipynb

@@ -1,5 +1,13 @@
 {
  "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Setup\n",
+    "Note: this notebook relies on unreleased Cirq features. If you want to try these features, make sure you install cirq via `pip install cirq --pre`."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -12,14 +20,14 @@
     "    import cirq\n",
     "except ImportError:\n",
     "    print(\"installing cirq...\")\n",
-    "    !pip install --quiet cirq\n",
+    "    !pip install --quiet cirq --pre\n",
     "    print(\"installed cirq.\")\n",
     "\n",
     "try:\n",
     "    import quimb\n",
     "except ImportError:\n",
     "    print(\"installing cirq-core[contrib]...\")\n",
-    "    !pip install --quiet 'cirq-core[contrib]'\n",
+    "    !pip install --quiet 'cirq-core[contrib]' --pre\n",
     "    print(\"installed cirq-core[contrib].\")"
    ]
   },