Enable testing of quimb-dependent notebook (#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 #6438 otherwise the
tensor_expectation_value() call in the last-cell causes out-of-memory error.

Fixes #6088

Author: pavoljuhas

cirq-core/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].\")"
    ]
   },

dev_tools/notebooks/isolated_notebook_test.py

@@ -42,6 +42,8 @@
 # Please, always indicate in comments the feature used for easier bookkeeping.
 
 NOTEBOOKS_DEPENDING_ON_UNRELEASED_FEATURES: List[str] = [
+    # Requires pinned quimb from #6438
+    'cirq-core/cirq/contrib/quimb/Contract-a-Grid-Circuit.ipynb',
     # Hardcoded qubit placement
     'docs/google/qubit-placement.ipynb',
     # get_qcs_objects_for_notebook
@@ -63,9 +65,6 @@
 # By default all notebooks should be tested, however, this list contains exceptions to the rule
 # please always add a reason for skipping.
 SKIP_NOTEBOOKS = [
-    # TODO(#6088) - enable notebooks below
-    'cirq-core/cirq/contrib/quimb/Contract-a-Grid-Circuit.ipynb',
-    # End of TODO(#6088)
     # skipping vendor notebooks as we don't have auth sorted out
     "**/aqt/*.ipynb",
     "**/azure-quantum/*.ipynb",