[build] chore: revert "bump transformer-engine to release_v2.16.post (#4536)"

[ko3n1g]

Background

This reverts commit 7c0968e (#4536), which bumped
transformer-engine to release_v2.16.post (b9d690e).

What changed

• Restores the transformer-engine pin to the prior commit d64bc14
  (release_v2.16_post) in pyproject.toml.
• Regenerates the matching embedded SHA in uv.lock.

Details

• pyproject.toml: transformer-engine @ git+...@b9d690e... → ...@d64bc14...
• uv.lock: 6 lines updated to match the reverted pin (lockfile embeds the source SHA).
• No commits between [build] chore: bump transformer-engine to release_v2.16.post #4536 and main touched these files, so the revert applies cleanly.

[claude]

LGTM — clean revert of the TE pin from b9d690e back to d64bc14. The pyproject.toml and uv.lock changes are consistent and no other files are touched.\n\nOne note: Per project policy, this PR should have the needs-more-tests label applied (and likely full-test-suite as well, since this is a TE pin change that touches CUDA kernels). Currently no labels are set.\n\n## Suggested test cases\n\nNo perf tests impacted.

[ko3n1g]

CI note (not blocking on this PR's content): the red Lint check / Nemo_CICD_Test here is a pre-existing breakage on main, not introduced by this revert.

pre-commit runs ruff --all-files, which flags F821 Undefined name for cu_seqlens, cu_seqlens_argmin, cu_seqlens_unpadded, cu_seqlens_unpadded_argmin in src/megatron/bridge/training/gpt_step.py:415-418. Those names are referenced in the accumulate_flops_metadata(...) call inside _forward_step_common, but get_batch only returns (tokens, labels, loss_mask, attention_mask, position_ids, packed_seq_metadata) — they are never bound in scope. This was introduced by #4511 (current main HEAD) and affects every open PR until main is fixed.

This revert touches only pyproject.toml and uv.lock; it cannot turn lint green on its own.

[ko3n1g]

/ok to test a7caf94