Add in-place reference counting test

ci_test/unit_tests/test_unit_inplace_ref_count.py

@@ -0,0 +1,36 @@
+import lbann
+import numpy as np
+import test_util
+import pytest
+
+
+@test_util.lbann_test(check_gradients=True)
+def test_inplace_ref_count():
+    # Prepare reference output
+    np.random.seed(20240430)
+    # Note: ReLU is not differentiable at 0, so we make sure values
+    # are away from 0.
+    num_samples = 24
+    sample_size = 48
+    samples = np.random.choice([-1.0, 1.0], size=(num_samples, sample_size))
+    samples += np.random.uniform(-0.5, 0.5, size=samples.shape)
+    ref = np.maximum(0, samples)
+
+    tester = test_util.ModelTester()
+
+    x_lbann = tester.inputs(samples)
+    reference = tester.make_reference(ref)
+
+    # LBANN implementation:
+    # The first relu will run in-place and then decref its inputs since it
+    # doesn't need them for backprop. The second relu will then do the same.
+    # If the in-place layer's output (same buffer as input) is properly
+    # reference counted, then it will not be freed before it is needed for the
+    # in-place layer's backprop.
+    x = lbann.Relu(x_lbann)
+    x = lbann.Relu(x)
+
+    # Set test loss
+    tester.set_loss(lbann.MeanSquaredError(x, reference))
+    tester.set_check_gradients_tensor(x)
+    return tester