Refactor CLI [7/N]: Move patching to compat and import transformers conditionally

[albertvillanova]

Move patching to _compat and import transformers conditionally:

Note that currently, importing transformers unconditionally, increases latency even if the patch is not necessary. See upstream issue:

• Lazy loading is not working properly transformers#44273

This PR refactors the way compatibility with older versions of the transformers and accelerate libraries is handled, specifically regarding the ParallelismConfig symbol. The patch moves the compatibility workaround for ParallelismConfig from trl/scripts/utils.py to a more appropriate location in trl/_compat.py, and improves its implementation by adding version checks and error handling.

Transformers/Accelerate Compatibility Improvements:

• Added a new function _patch_transformers_parallelism_config in trl/_compat.py to ensure transformers.training_args.ParallelismConfig is defined only when both transformers<4.57.0 and accelerate<1.10.1 are installed, preventing NameError during type hint resolution. The function includes error handling and a detailed docstring explaining its purpose and upstream context.
• Removed the previous unconditional workaround function _ensure_transformers_parallelism_config from trl/scripts/utils.py, as this logic is now handled (with improvements) in trl/_compat.py.