Fail gracefully in lobpcg_test if matplotlib isn't installed.

There isn't yet a matplotlib that supports NumPy 2.0, so we need to support running tests without it.

PiperOrigin-RevId: 621228727

Author: hawkinsp

tests/lobpcg_test.py

@@ -288,7 +288,10 @@ def _debug_plots(self, X, eigs, info, matrix_name, lobpcg_debug_plot_dir):
     # We import matplotlib lazily because (a) it's faster this way, and
     # (b) concurrent imports of matplotlib appear to trigger some sort of
     # collision on the matplotlib cache lock on Windows.
-    from matplotlib import pyplot as plt
+    try:
+      from matplotlib import pyplot as plt
+    except (ModuleNotFoundError, ImportError):
+      return  # If matplotlib isn't available, don't emit plots.
 
     os.makedirs(lobpcg_debug_plot_dir, exist_ok=True)
     clean_matrix_name = _clean_matrix_name(matrix_name)