fix: guard against None chat_template in _post_process_chat_template

[yeyu-nvidia]

Problem

When training with a model that has no chat_template in its tokenizer (e.g. base Llama-3.2 models), _post_process_chat_template() crashes:

AttributeError: 'NoneType' object has no attribute 'replace'

The DeepSeek WAR at the top of _post_process_chat_template called .replace() directly on self.tokenizer.chat_template without checking for None first.

Fixes NVBug 6120958

Fix

Add an early return when chat_template is None. The existing check at line 164 (if self.tokenizer.chat_template is None: raise ValueError) still provides a clear error message if no valid template is available after post-processing.

Summary by CodeRabbit

• Bug Fixes
  • Fixed a crash in chat template processing that occurred when a chat template configuration was not set, improving system stability and reliability during initialization.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 0fb9ead and 378d3e9.

📒 Files selected for processing (1)

• modelopt/torch/utils/plugins/transformers_dataset.py

🚧 Files skipped from review as they are similar to previous changes (1)

• modelopt/torch/utils/plugins/transformers_dataset.py

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📝 Walkthrough

Walkthrough

A null guard is added to the _post_process_chat_template method to check if self.tokenizer.chat_template is None before attempting string operations on it, preventing null reference errors when the template is unset.

Changes

Cohort / File(s)	Summary
Null Safety Fix modelopt/torch/utils/plugins/transformers_dataset.py	Added None guard in _post_process_chat_template to prevent null dereference when self.tokenizer.chat_template is unset.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: adding a None guard for chat_template in the _post_process_chat_template method to prevent null dereference errors.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Security Anti-Patterns	✅ Passed	Security audit found no torch.load(weights_only=False), numpy.load(allow_pickle=True), hardcoded trust_remote_code=True, eval/exec on untrusted input, # nosec comments, or non-permissive license dependencies.

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• Commit unit tests in branch fix/chat-template-none-guard

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[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.97%. Comparing base (077e29a) to head (378d3e9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...delopt/torch/utils/plugins/transformers_dataset.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1371      +/-   ##
==========================================
+ Coverage   76.48%   76.97%   +0.48%     
==========================================
  Files         471      471              
  Lines       50487    50489       +2     
==========================================
+ Hits        38617    38862     +245     
+ Misses      11870    11627     -243     

Flag	Coverage Δ
examples	41.58% <50.00%> (+10.82%)	⬆️
gpu	59.71% <0.00%> (-0.44%)	⬇️
regression	14.91% <0.00%> (+0.19%)	⬆️
unit	52.78% <0.00%> (-0.01%)	⬇️

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