test: illustrate int4 marlin kernel bug

Author: dacorvo

test/tensor/weights/optimized/test_marlin_int4_weight_qbits_tensor.py

@@ -118,3 +118,33 @@ def test_marlin_int4_weight_qbits_tensor_linear(batch_size, tokens, embeddings,
     qout = torch.nn.functional.linear(inputs, marlin_qweight, bias)
     out = torch.nn.functional.linear(inputs, qbt.dequantize(), bias)
     assert_similar(out, qout)
+
+
+@pytest.mark.skipif(not torch.cuda.is_available(), reason="CUDA not available")
+@pytest.mark.parametrize("tokens", [16, 32, 33])
+def test_marlin_int4_weight_qbits_tensor_linear_bug(tokens):
+    device = torch.device("cuda")
+    dtype = torch.float16
+    weight_qtype = qint4
+    group_size = 128
+    in_features = 4096
+    out_features = 2048
+    inputs = torch.rand((tokens, in_features), dtype=dtype, device=device)
+    # Create a MarlinInt4WeightQBitsTensor from a QBitsTensor on CUDA
+    qbt = random_qweight((out_features, in_features), weight_qtype, dtype, group_size=group_size, device=torch.device("cuda"))
+    marlin_qweight = MarlinInt4WeightQBitsTensor(
+        qtype=qbt.qtype,
+        axis=qbt.axis,
+        group_size=qbt._group_size,
+        size=qbt.size(),
+        stride=qbt.stride(),
+        data=qbt._data.unpack(),
+        scale=qbt._scale,
+        shift=qbt._shift,
+    )
+    qout = torch.nn.functional.linear(inputs, marlin_qweight, bias=None)
+    out = torch.nn.functional.linear(inputs, qbt.dequantize(), bias=None)
+    max_val = out.abs().max()
+    max_err = (out - qout).abs().max()
+    print(max_val, max_err)
+    assert max_err / max_val < 1e-2