Move tests to src folder

src/tests/conftest.py

src/tests/datasets/test_custom_dataset.py

src/tests/datasets/test_grammar_datasets.py

src/tests/datasets/test_samsum_datasets.py

src/tests/test_batching.py

src/tests/test_chat_completion.py

src/tests/test_finetuning.py

src/tests/test_finetuning_data_formatter.py

src/tests/test_sampler.py

src/tests/test_train_utils.py