Fix client import error by vendoring transformers constants

[corbt]

Summary

• Fixes ART client missing dependencies #230 - ART client missing dependencies error
• Vendors transformers enum constants into art/dev/model.py to remove the hard dependency
• Removes fastapi dependency from art/errors.py by using custom exception classes

Context

Issue #230 reported that importing art.Model on the client side fails with ModuleNotFoundError: No module named 'torch' and ModuleNotFoundError: No module named 'transformers'.

This was caused by PR #194 which moved training/inference dependencies to a [backend] group but didn't account for:

1. art.dev importing transformers at the module level
2. art.errors importing fastapi's HTTPException

My apologies for not testing the client import path properly in that PR.

Solution

This PR takes a two-pronged approach to fix the client import issues:

1. Vendor transformers constants

Instead of using lazy imports (which would complicate the codebase), this PR vendors all the transformers enum constants that art.dev.model needs:

• OptimizerNames - from transformers.training_args
• DebugOption - from transformers.debug_utils
• IntervalStrategy - from transformers.trainer_utils
• SaveStrategy - alias for IntervalStrategy
• HubStrategy - from transformers.trainer_utils
• SchedulerType - from transformers.trainer_utils
• FSDPOption - from transformers.trainer_utils

Each vendored constant includes a comment indicating its source for future reference.

2. Remove fastapi dependency

Replaced fastapi's HTTPException with a custom ARTError base class that has the same status_code and detail attributes. This allows the exceptions to work with FastAPI's exception handlers while removing the hard dependency on fastapi for client imports.

Test plan

• Tested in test/check_deps environment that has only base ART dependencies
• Verified import art and art.Model(...) work without transformers installed
• Verified art.dev.EngineArgs() can be instantiated without importing torch/transformers
• All vendored enums maintain exact same values as transformers originals
• Custom exceptions maintain compatibility with FastAPI handlers

@bradhilton Could you please review and make sure this approach looks reasonable? Happy to adjust if you'd prefer a different solution.

🤖 Generated with Claude Code