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.

kda/_fla/utils/_testing.py

@@ -0,0 +1,41 @@
+# Copyright (c) 2023-2026, Songlin Yang, Yu Zhang, Zhiyuan Li
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+# For a list of all contributors, visit:
+#   https://github.com/fla-org/flash-linear-attention/graphs/contributors
+
+import logging
+import warnings
+
+import torch
+
+from ._config import FLA_CI_ENV
+
+logger = logging.getLogger(__name__)
+
+
+def get_abs_err(x, y):
+    return (x.detach() - y.detach()).flatten().abs().max().item()
+
+
+def get_err_ratio(x, y):
+    err = (x.detach() - y.detach()).flatten().square().mean().sqrt().item()
+    base = (x.detach()).flatten().square().mean().sqrt().item()
+    return err / (base + 1e-8)
+
+
+def assert_close(prefix, ref, tri, ratio, warning=False, err_atol=1e-6):
+    abs_atol = get_abs_err(ref, tri)
+    error_rate = get_err_ratio(ref, tri)
+    msg = f"{prefix:>16} diff: {abs_atol:.6f} ratio: {error_rate:.6f}"
+    logger.info(msg)
+    if abs_atol <= err_atol:
+        return
+    assert not torch.isnan(ref).any(), f"{prefix}: NaN detected in ref"
+    assert not torch.isnan(tri).any(), f"{prefix}: NaN detected in tri"
+    if warning or (FLA_CI_ENV and (error_rate < 0.01 or abs_atol <= 0.3)):
+        if error_rate > ratio:
+            warnings.warn(msg)
+    else:
+        assert error_rate < ratio, msg