Initial K3 snapshot: 0.5B KDA/MLA/MoE train path

Standalone tree split from LLMRL/projects/kda. Includes Triton dt_bias
backward fix, train_k3 --preset 0.5b, SFT, Docker runtime, and tests.

tests/correctness/test_chunkwise.py

@@ -0,0 +1,47 @@
+"""L2: chunked naive vs L1 naive (fwd) + gradcheck."""
+import torch
+
+from kda.ops.reference.chunkwise import naive_chunk_kda
+from kda.ops.reference.recurrent import naive_kda
+
+
+def test_fwd_matches_naive():
+    """chunked vs naive fwd, atol=1e-4."""
+    B, T, H, HV, K, V = 2, 32, 2, 4, 8, 8
+    torch.manual_seed(1)
+    q = torch.randn(B, T, H, K, dtype=torch.float64)
+    k = torch.randn(B, T, H, K, dtype=torch.float64)
+    v = torch.randn(B, T, HV, V, dtype=torch.float64)
+    g = torch.randn(B, T, HV, K, dtype=torch.float64) * 0.1
+    beta = torch.rand(B, T, HV, dtype=torch.float64)
+
+    o_ref, _ = naive_kda(q, k, v, g, beta, output_final_state=True)
+    o_chk, _ = naive_chunk_kda(q, k, v, g, beta,
+                                output_final_state=True, chunk_size=8)
+
+    diff = (o_ref - o_chk).abs().max().item()
+    assert diff < 1e-4, f"chunk vs naive fwd max diff {diff:.2e} > 1e-4"
+    print(f"L2 fwd-vs-naive: PASSED (max diff {diff:.2e})")
+
+
+def test_gradcheck():
+    """L2 chunked gradcheck atol=1e-4 (allowing rtol 1e-3 for triangular path)."""
+    B, T, H, HV, K, V = 2, 16, 2, 4, 4, 4
+    torch.manual_seed(2)
+    q = torch.randn(B, T, H, K, dtype=torch.float64, requires_grad=True)
+    k = torch.randn(B, T, H, K, dtype=torch.float64, requires_grad=True)
+    v = torch.randn(B, T, HV, V, dtype=torch.float64, requires_grad=True)
+    g = torch.randn(B, T, HV, K, dtype=torch.float64, requires_grad=True) * 0.1
+    beta = torch.rand(B, T, HV, dtype=torch.float64, requires_grad=True)
+
+    assert torch.autograd.gradcheck(
+        lambda q, k, v, g, b: naive_chunk_kda(q, k, v, g, b, chunk_size=4)[0],
+        (q, k, v, g, beta),
+        eps=1e-6, atol=1e-4, rtol=1e-3,
+    ), "L2 gradcheck 失败"
+    print("L2 gradcheck: PASSED")
+
+
+if __name__ == "__main__":
+    test_fwd_matches_naive()
+    test_gradcheck()