Add nn.Sequential: callable ModuleList that chains forward calls

[justinchuby]

Usage PR: https://github.com/justinchuby/onnx-genai-models/pull/19

nn.Sequential mirrors torch.nn.Sequential — children are registered
with numeric keys and forward() passes each child's output as the
next child's input.

Key design decisions:

• Subclasses ModuleList for registration, indexing, iteration
• Overrides _set_name to keep children with simple '0', '1' names
  (avoids double-prefixing since call already pushes the parent name)
• Raises RuntimeError on empty Sequential

This enables matching HF diffusers naming conventions (e.g.
nn.Sequential(SiLU(), Linear(...)) producing 'mod.1.weight') without
needing preprocess_weights renames.

[Copilot]

Pull request overview

This PR adds nn.Sequential, a callable container that chains forward calls through its child modules, mirroring PyTorch's torch.nn.Sequential. This enables cleaner model definitions and matches HuggingFace diffusers naming conventions without requiring weight preprocessing.

Changes:

• Implements Sequential class that subclasses ModuleList and chains forward calls
• Updates import statements across the codebase to use aliased builder module imports
• Adds comprehensive test coverage for Sequential functionality

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
onnxscript/nn/_sequential.py	New file implementing Sequential container with forward chaining and custom naming behavior
onnxscript/nn/_module_test.py	Adds four test cases covering Sequential's forward chaining, parameter naming, mixed modules, and empty container handling
onnxscript/nn/init.py	Exports Sequential in public API
onnxscript/nn/_module.py	Refactors builder imports to use aliased form
onnxscript/nn/_module_list.py	Refactors builder imports to use aliased form
onnxscript/nn/_parameter.py	Refactors builder imports to use aliased form

[codecov]

Codecov Report

❌ Patch coverage is 96.08696% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.66%. Comparing base (f99fa7d) to head (02ec841).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/nn/_sequential.py	74.07%	6 Missing and 1 partial ⚠️
onnxscript/nn/_module_test.py	98.94%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2823      +/-   ##
==========================================
+ Coverage   71.48%   71.66%   +0.18%     
==========================================
  Files         237      238       +1     
  Lines       28645    28864     +219     
  Branches     2841     2849       +8     
==========================================
+ Hits        20476    20686     +210     
- Misses       7199     7207       +8     
- Partials      970      971       +1     

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