Exclude finetuning datasets from the `pretrain.py` arguments

Nobody has claimed this yet.

The `pretrain.py` script lists the Alpaca dataset and all other finetuning datasets, but I don't think they are supported for finetuning.

E.g.,

```bash
python litgpt/pretrain.py \
--data litgpt.data.Alpaca2k \
--model_name tiny-llama-1.1b \
--tokenizer_dir checkpoints/TinyLlama/TinyLlama-1.1B-intermediate-step-1431k-3T/
```

```
File "litgpt/pretrain.py", line 381, in
CLI(setup)
File "/teamspace/studios/this_studio/lit-gpt/litgpt/utils.py", line 399, in CLI
return CLI(*args, **kwargs)
File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.8/site-packages/jsonargparse/_cli.py", line 96, in CLI
return _run_component(components, cfg_init)
File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.8/site-packages/jsonargparse/_cli.py", line 193, in _run_component
return component(**cfg)
File "litgpt/pretrain.py", line 90, in setup
main(fabric, devices, seed, resume, config, data, out_dir, tokenizer_dir, tokenizer, train, eval)
File "litgpt/pretrain.py", line 155, in main
fit(fabric, devices, state, train_dataloader, val_dataloader, out_dir, tokenizer_dir, train, eval)
File "litgpt/pretrain.py", line 175, in fit
validate(fabric, model, val_dataloader, max_iters=2) # sanity check
File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.8/site-packages/torch/utils/_contextlib.py", line 115, in decorate_context
return func(*args, **kwargs)
File "litgpt/pretrain.py", line 303, in validate
input_ids = batch[:, 0 : model.max_seq_length].contiguous().long()
TypeError: unhashable type: 'slice'
```

I think we should exclude those finetuning datasets from the pretraining args?

Start in litgpt/pretrain.py at the data argument definitions and compare the listed datasets with the pretraining data interface. Run the example pretrain command to reproduce the reported validation error; done means finetuning-only datasets are no longer offered as pretraining arguments and supported pretraining data still works.

Written by the indexing model from the issue text.