Pass train_dtype into _load_model_memory_efficient

[RobotSail]

Summary

• Adds train_dtype as an explicit parameter to _load_model_memory_efficient instead of extracting it from base_kwargs internally
• Removes the now-unnecessary torch_dtype lookup and validation from inside the function
• Updates the call site in from_pretrained to extract and validate torch_dtype before passing it as train_dtype
• Adds two new tests: one verifying train_dtype overrides base_kwargs["torch_dtype"], and one verifying it works when base_kwargs has no torch_dtype

Closes #34

Test plan

• ruff check passes
• ruff format --check passes
• Existing test_memory_efficient_loading_calls_alignment_hook updated with new parameter
• New test_memory_efficient_loading_uses_train_dtype verifies override behavior
• New test_memory_efficient_loading_fallback_when_base_kwargs_missing_dtype verifies no-dtype-in-kwargs case

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c243ffe7-b9c4-485b-90f4-259600fe8f8a

📥 Commits

Reviewing files that changed from the base of the PR and between e3db6da and 700e288.

📒 Files selected for processing (2)

• src/mini_trainer/osft_utils.py
• tests/test_osft.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/mini-trainer-dev/cec043a1

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[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mini_trainer/osft_utils.py	40.00%	3 Missing ⚠️

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