Refactor causal LM tests to inherit from base classes
- Updated 5 models to inherit from CausalLMModelTester and CausalLMModelTest
- Models updated: bamba, biogpt, bloom, codegen, cohere
- Removed redundant methods that are now inherited from base classes
- Added required attributes (base_model_class, causal_lm_class) where needed
- Fixed model-specific issues:
- CodeGen: Set use_token_type_ids=False to avoid parameter conflicts
- All models: Removed redundant test methods already in base classes
- All tests passing except bloom's test_bloom_model_past_large_inputs (alibi-specific issue)
