activation_checkpointing: default num_layers to None so configure() assert fires

deepspeed/runtime/activation_checkpointing/checkpointing.py

@@ -1019,7 +1019,7 @@ def _configure_defaults():
 
     PARTITION_ACTIVATIONS = False
     CONTIGUOUS_CHECKPOINTING = False
-    num_layers = False
+    num_layers = None
     CPU_CHECKPOINT = False
     SYNCHRONIZE = False
     PROFILE_TIME = False

tests/unit/runtime/activation_checkpointing/test_activation_checkpointing.py

@@ -309,3 +309,45 @@ def __init__(self):
         assert model._is_checkpointable([layers[0]]) == True  # ParallelTransformerLayerPipe
         assert model._is_checkpointable([layers[1]]) == True  # GMLPBlock
         assert model._is_checkpointable([layers[2]]) == False  # Linear layer
+
+
+def test_configure_with_contiguous_checkpointing_requires_num_checkpoints():
+    # Regression: ``_configure_defaults`` previously initialized ``num_layers``
+    # to ``False`` while the assert below uses ``is not None``; ``False is not
+    # None`` is True, so the missing-config assert silently passed and a
+    # cryptic ``IndexError`` surfaced later from ``range(num_layers)``. With
+    # the default switched to ``None`` (matching the module-level default),
+    # the helpful assert message fires at the configure() call site.
+    #
+    # ``configure()`` mutates module globals before raising, so snapshot and
+    # restore them around the call to avoid order-dependent failures in other
+    # activation-checkpointing tests sharing the same pytest worker.
+    cp = deepspeed.checkpointing
+    saved = (
+        cp.PARTITION_ACTIVATIONS,
+        cp.CONTIGUOUS_CHECKPOINTING,
+        cp.num_layers,
+        cp.CPU_CHECKPOINT,
+        cp.SYNCHRONIZE,
+        cp.PROFILE_TIME,
+        cp.mpu,
+        cp.deepspeed_checkpointing_enabled,
+    )
+    try:
+        with pytest.raises(AssertionError, match="number of layers"):
+            deepspeed.checkpointing.configure(
+                mpu_=None,
+                partition_activations=True,
+                contiguous_checkpointing=True,
+            )
+    finally:
+        (
+            cp.PARTITION_ACTIVATIONS,
+            cp.CONTIGUOUS_CHECKPOINTING,
+            cp.num_layers,
+            cp.CPU_CHECKPOINT,
+            cp.SYNCHRONIZE,
+            cp.PROFILE_TIME,
+            cp.mpu,
+            cp.deepspeed_checkpointing_enabled,
+        ) = saved