Retire test_multi_gpu_data_parallel_forward (#48508)
`nn.DataParallel` cannot replicate or gather the forward patterns and
output types modern Transformers models use, so this common test measures
PyTorch's legacy DP path rather than model correctness.
DataParallel is a Trainer feature, not a per-model contract, and it keeps
its coverage there (tests/trainer/test_trainer.py). This removes the common
test and all 66 overrides; no `src/` behavior changes.
