Keep up with quimb-1.5.0 (#6087)

As of quimb-1.5.0 `TensorNetwork.contract(inplace=True)` returns
TensorNetwork instead of scalar.  Convert to scalar if needed.

Also correct CI issues that showed for testing notebooks w/r to the cirq head here:

* Fix misleading unit test name by renaming
  `test_notebooks_against_released_cirq` --> `test_notebooks_against_cirq_head`.
  Note: Notebooks are tested with released Cirq in isolated_notebook_test.py.

* Append to PYTHONPATH when tested notebooks pip-install any packages.
  Prefer use of development Cirq sources in `test_notebooks_against_cirq_head`.

* Disable isolated CI test of Contract-a-Grid-Circuit.ipynb
  Requires next cirq release.

cirq-core/cirq/contrib/quimb/state_vector.py

@@ -177,6 +177,8 @@ def tensor_expectation_value(
     if ram_gb > max_ram_gb:
         raise MemoryError(f"We estimate that this contraction will take too much RAM! {ram_gb} GB")
     e_val = tn.contract(inplace=True)
+    if isinstance(e_val, qtn.TensorNetwork):
+        e_val = e_val.item()
     assert e_val.imag < tol
     assert cast(complex, pauli_string.coefficient).imag < tol
     return e_val.real * pauli_string.coefficient

dev_tools/notebooks/isolated_notebook_test.py

@@ -62,6 +62,9 @@
 # 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",

dev_tools/notebooks/notebook_test.py

@@ -73,8 +73,12 @@ def require_packages_not_changed():
 def env_with_temporary_pip_target():
     """Setup system environment that tells pip to install packages to a temporary directory."""
     with tempfile.TemporaryDirectory(suffix='-notebook-site-packages') as tmpdirname:
+        # Note: We need to append tmpdirname to the PYTHONPATH, because PYTHONPATH may
+        # already point to the development sources of Cirq (as happens with check/pytest).
+        # Should some notebook pip-install a stable version of Cirq to tmpdirname,
+        # it would appear in PYTHONPATH after the development Cirq.
         pythonpath = (
-            f'{tmpdirname}{os.pathsep}{os.environ["PYTHONPATH"]}'
+            f'{os.environ["PYTHONPATH"]}{os.pathsep}{tmpdirname}'
             if 'PYTHONPATH' in os.environ
             else tmpdirname
         )
@@ -84,7 +88,7 @@ def env_with_temporary_pip_target():
 
 @pytest.mark.slow
 @pytest.mark.parametrize("notebook_path", filter_notebooks(list_all_notebooks(), SKIP_NOTEBOOKS))
-def test_notebooks_against_released_cirq(
+def test_notebooks_against_cirq_head(
     notebook_path, require_packages_not_changed, env_with_temporary_pip_target
 ):
     """Test that jupyter notebooks execute.