Megatron @jit_fuser Fails: 'Unknown type constructor Sequence'

torch.jit.script on a Megatron @jit_fuser-decorated function fails with 'Unknown type constructor Sequence' on newer torch. TorchScript can't resolve typing.Sequence as used inside the fused function. Upgrade Megatron (the fix moved the vocab-range lookup out of the JITed function) or replace the unsupported annotation.

Quick answer

torch.

Environment#megatron-lm#torchscript#jit_fuser#typing#sequence#environment

What this failure is

Megatron @jit_fuser Fails: 'Unknown type constructor Sequence' is a Environment failure seen during ML training runs. torch.jit.script on a Megatron @jit_fuser-decorated function fails with 'Unknown type constructor Sequence' on newer torch. TorchScript can't resolve typing.Sequence as used inside the fused function. Upgrade Megatron (the fix moved the vocab-range lookup out of the JITed function) or replace the unsupported annotation. Common tags: Megatron Lm, Torchscript, Jit_fuser, Typing.

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Why it happens (the mechanism)

TorchScript cannot resolve the typing.Sequence constructor as used in the fused function under newer torch. The vocab-range lookup carried an annotation TorchScript no longer supports inside the scripted body. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.

What you'll observe

torch.jit.script fails compiling a Megatron fused function
Startup crashes before training on newer torch
@jit_fuser decorated function rejected by TorchScript

Common symptoms and what they mean

Symptom	Why it happens
RuntimeError: Unknown type constructor Sequence	TorchScript cannot resolve the typing.Sequence constructor as used in the fused function under newer torch
Traceback through torch/jit/_script.py from a @jit_fuser function	The vocab-range lookup carried an annotation TorchScript no longer supports inside the scripted body
Occurs with newer torch versions	TorchScript cannot resolve the typing.Sequence constructor as used in the fused function under newer torch

Which systems are affected

Megatron-LM fused functions using @jit_fuser / torch.jit.script
Newer torch where TorchScript rejects typing.Sequence
Functions with a vocab-range lookup inside the JITed body

How to confirm this is the problem

Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.

✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
✓Verified signal present: RuntimeError: Unknown type constructor Sequence
✓Verified signal present: Traceback through torch/jit/_script.py from a @jit_fuser function
✓Verified signal present: Occurs with newer torch versions
✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

The fix and the prevention pattern

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Related failures to investigate next

Python Path Conflict often co-occurs with this failure because both share underlying assumptions about environment; if you fix this entry but still see drift, validate the related primary cause next. python path conflict →

Env Variable Not Set often co-occurs with this failure because both share underlying assumptions about environment; if you fix this entry but still see drift, validate the related neighboring cause next. env variable not set →

Transformers Version Mismatch often co-occurs with this failure because both share underlying assumptions about environment; if you fix this entry but still see drift, validate the related neighboring cause next. transformers version mismatch →

Root cause

TorchScript cannot resolve the typing.Sequence constructor as used in the fused function under newer torch
The vocab-range lookup carried an annotation TorchScript no longer supports inside the scripted body

The fix and how to prevent it

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References

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