activation_checkpointing: default num_layers to None so configure() assert fires (#8041)

## Summary

Calling
`deepspeed.checkpointing.configure(contiguous_checkpointing=True,
partition_activations=True)` *without* setting `num_checkpoints` (or
otherwise providing a layer count) is supposed to fail fast with the
assert at
`deepspeed/runtime/activation_checkpointing/checkpointing.py:1108`:

```python
if CONTIGUOUS_CHECKPOINTING:
    assert num_layers is not None, "Must specify the number of layers with contiguous memory checkpointing"
```

That assert never fires because `_configure_defaults()` (called inside
`configure()` at line 1079) initialized:

```python
num_layers = False
```

`False is not None` is `True`, so the assert silently passes. The user
instead hits a much later cryptic `IndexError` from `range(num_layers)`
(lines 399 / 406) or a 0-element allocation from `numel * num_layers`
(lines 457 / 461).

The module-level default at line 46 is already `num_layers = None`, and
every other path that sets it (`set_num_layers`,
`config.number_checkpoints`) assigns an integer — only
`_configure_defaults` used `False`, which looks like a copy-paste from
the surrounding `PARTITION_ACTIVATIONS = False` etc. block.

## Fix

```diff
-    num_layers = False
+    num_layers = None
```

One-character change. No callers compare `num_layers` to `False`
(downstream uses are `range(num_layers)` and `numel * num_layers`, both
requiring an int), so the only path this changes is the broken one:
users now get the documented "Must specify the number of layers" assert
instead of a downstream `IndexError`.

## Test

Adds
`test_configure_with_contiguous_checkpointing_requires_num_checkpoints`
to the existing
`tests/unit/runtime/activation_checkpointing/test_activation_checkpointing.py`
(consolidating per `AGENTS.md`, not a new file). It calls
`configure(contiguous_checkpointing=True, partition_activations=True)`
and asserts the expected `AssertionError` matches `"number of layers"`.
On `main` the assert silently passes and the test fails; with this
change it passes.

## CI/lint

- `pre-commit run --files <changed files>`: all hooks pass (yapf,
flake8, `check-torchdist`, `check-license`, codespell,
`check-torchcuda`).
- DCO `Signed-off-by` on the commit.
- No competing PR (`gh pr list --search "num_layers checkpointing OR
_configure_defaults OR contiguous_checkpointing assert"`).

---------

Signed-off-by: Kymi808 <zeng.kyle13@gmail.com>
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
Co-authored-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>

Author: 3 people

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