Added backprop test for Linear8bitLt and LinearFP4.

tests/test_modules.py

@@ -375,7 +375,7 @@ def test_linear8bitlt_accumulated_gradient():
 @pytest.mark.parametrize("threshold", [0.0, 2.0])
 @pytest.mark.parametrize("memory_efficient_backward", [False])
 def test_linear8bitlt_no_fp16_weights(threshold, memory_efficient_backward):
-    l1 = ( bnb.nn.Linear8bitLt( 32, 64, threshold=threshold, has_fp16_weights=False, memory_efficient_backward=memory_efficient_backward).cuda().half())
+    l1 = (bnb.nn.Linear8bitLt( 32, 64, threshold=threshold, has_fp16_weights=False, memory_efficient_backward=memory_efficient_backward).cuda().half())
     assert l1.weight.dtype == torch.int8
 
     l1.eval()
@@ -506,3 +506,41 @@ def test_linear_kbit_fp32_bias(module):
         o1 = l1(b1)
         assert l1.bias is None
 
+@pytest.mark.skipif(not torch.cuda.is_available(), reason="this test requires a GPU")
+@pytest.mark.parametrize("module", [bnb.nn.Linear8bitLt, bnb.nn.LinearFP4], ids=['Int8Lt', 'FP4'])
+def test_kbit_backprop(module):
+    b = 17
+    dim1 = 37
+    dim2 = 83
+
+    ref = nn.Sequential(*[torch.nn.Linear(dim1, dim2), torch.nn.Linear(dim2, 10)])
+    ref[1].weight.requires_grad = False
+    kbit = nn.Sequential(*[torch.nn.Linear(dim1, dim2), module(dim2, 10)])
+    kbit[0].weight.detach().copy_(ref[0].weight)
+    kbit[1].weight.detach().copy_(ref[1].weight)
+    kbit[0].bias.detach().copy_(ref[0].bias)
+    kbit[1].bias.detach().copy_(ref[1].bias)
+    ref = ref.half().cuda()
+    kbit = kbit.half().cuda()
+
+    for i in range(100):
+        batch = torch.randn(b, dim1).half().cuda()
+        out1 = ref(batch)
+        out2 = kbit(batch)
+        out1.mean().backward()
+        out2.mean().backward()
+
+        grad1 = ref[0].weight.grad
+        grad2 = kbit[0].weight.grad
+        bgrad1 = ref[0].bias.grad
+        bgrad2 = kbit[0].bias.grad
+
+        torch.testing.assert_allclose(grad1, grad2, atol=0.008, rtol=0.05)
+        torch.testing.assert_allclose(bgrad1, bgrad2, atol=0.008, rtol=0.05)
+        ref.zero_grad()
+        kbit.zero_grad()
+
+        assert kbit[0].weight.grad.sum().item() == 0
+        assert kbit[0].bias.grad.sum().item() == 0
+
+