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/_config.py

@@ -0,0 +1,17 @@
+# 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 os
+
+FLA_CI_ENV = os.getenv("FLA_CI_ENV") == "1"
+FLA_CACHE_RESULTS = os.getenv('FLA_CACHE_RESULTS', '1') == '1'
+
+FLA_DISABLE_TENSOR_CACHE = os.getenv('FLA_DISABLE_TENSOR_CACHE', '0') == '1'
+try:
+    FLA_TENSOR_CACHE_SIZE = int(os.getenv('FLA_TENSOR_CACHE_SIZE', "4"))
+except ValueError:
+    FLA_TENSOR_CACHE_SIZE = 4