LR scheduler can result in a division by 0

Nobody has claimed this yet.

If `--train.max_steps` is equal to `--train.lr_warmup_steps` then the `T_max` will result in a division by 0 https://github.com/Lightning-AI/litgpt/blob/6fd737d3da240a67f4acb7a3ce733fa2e67538a4/litgpt/finetune/lora.py#L385

```python
[rank0]: Traceback (most recent call last):
[rank0]: File "/home/carlos/nightly-env/bin/litgpt", line 8, in
[rank0]: sys.exit(main())
[rank0]: File "/home/carlos/lit-parrot/litgpt/__main__.py", line 143, in main
[rank0]: fn(**kwargs)
[rank0]: File "/home/carlos/lit-parrot/litgpt/finetune/lora.py", line 143, in setup
[rank0]: fabric.launch(main, devices, seed, config, data, checkpoint_dir, out_dir, train, eval)
[rank0]: File "/home/carlos/lightning/src/lightning/fabric/fabric.py", line 866, in launch
[rank0]: return self._wrap_and_launch(function, self, *args, **kwargs)
[rank0]: File "/home/carlos/lightning/src/lightning/fabric/fabric.py", line 951, in _wrap_and_launch
[rank0]: return launcher.launch(to_run, *args, **kwargs)
[rank0]: File "/home/carlos/lightning/src/lightning/fabric/strategies/launchers/subprocess_script.py", line 107, in launch
[rank0]: return function(*args, **kwargs)
[rank0]: File "/home/carlos/lightning/src/lightning/fabric/fabric.py", line 957, in _wrap_with_setup
[rank0]: return to_run(*args, **kwargs)
[rank0]: File "/home/carlos/lit-parrot/litgpt/finetune/lora.py", line 196, in main
[rank0]: fit(
[rank0]: File "/home/carlos/lit-parrot/litgpt/finetune/lora.py", line 291, in fit
[rank0]: scheduler.step()
[rank0]: File "/home/carlos/nightly-env/lib/python3.10/site-packages/torch/optim/lr_scheduler.py", line 838, in step
[rank0]: scheduler.step(0)
[rank0]: File "/home/carlos/nightly-env/lib/python3.10/site-packages/torch/optim/lr_scheduler.py", line 187, in step
[rank0]: values = self._get_closed_form_lr()
[rank0]: File "/home/carlos/nightly-env/lib/python3.10/site-packages/torch/optim/lr_scheduler.py", line 1029, in _get_closed_form_lr
[rank0]: return [
[rank0]: File "/home/carlos/nightly-env/lib/python3.10/site-packages/torch/optim/lr_scheduler.py", line 1032, in
[rank0]: * (1 + math.cos(math.pi * self.last_epoch / self.T_max))
[rank0]: ZeroDivisionError: float division by zero
```

Litgpt should validate that this doesn't happen

Start in litgpt/finetune/lora.py at the scheduler setup around line 385, then follow the training path shown in the traceback. Validate the --train.max_steps and --train.lr_warmup_steps combination so equal values cannot produce a zero T_max, and verify that this configuration no longer raises ZeroDivisionError.

Written by the indexing model from the issue text.