fix: pin megatron-core to 0.16.1 in gpu-megatron CI

mcore 0.17.0 requires nvidia-resiliency-ext with get_write_results_queue
which is not available in the pytorch:26.01-py3 CI container.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenhan Yu <chenhany@nvidia.com>

Author: ChenhanYu

tox.ini

@@ -78,7 +78,7 @@ commands =
 [testenv:cuda13-gpu-megatron]
 commands_pre =
     # Install deps here so that it gets installed even in --current-env
-    pip install -U megatron-core
+    pip install -U megatron-core==0.16.1
     pip install --no-build-isolation git+https://github.com/state-spaces/mamba.git
     pip install --no-build-isolation git+https://github.com/Dao-AILab/causal-conv1d.git
     pip install -e .[hf,dev-test]