[Bug] AssertionError: neat_packing + context_parallel alignment mismatch (sub-sequence length not aligned to 2 * cp_size)

When training with Megatron Trainer’s backend in LLaMA-Factory, enabling both neat_packing and context_parallel_size (CP) results in an AssertionError: neat_packing + context_parallel alignment mismatch (sub-sequence length not aligned to 2 * cp_size)

config:
........
cut_off_len: 32768
sequence_packin: true
context_parallel_size: 4
.........

Start at the Megatron Trainer backend path that handles neat_packing with context_parallel_size; reproduce the failure using cut_off_len 32768, sequence packing, and context_parallel_size 4. Trace the reported alignment assertion and establish the intended behavior for sub-sequence lengths; done means the configuration no longer raises the mismatch and a regression test covers it.

Written by the indexing model from the issue text.