fix: guard target-param-data-ratio against zero to avoid ZeroDivision…

[suraj-self]

This PR adds a sanity check for the --target-param-data-ratio argument. Fix for #578

• Prevents ZeroDivisionError when the user accidentally provides 0.
• Ensures the script fails fast with a clear message: "target-param-data-ratio must be positive (or -1 to disable)".
• Maintains compatibility with the -1 flag used to disable scaling-based iterations.

Logs

(nanochat) ss@ss-Predator-PH315-53:~/nanochat$ python -m scripts.base_train   --depth 1   --aspect-ratio 1   --target-param-data-ratio 0

                                                       █████                █████
                                                      ░░███                ░░███
     ████████    ██████   ████████    ██████   ██████  ░███████    ██████  ███████
    ░░███░░███  ░░░░░███ ░░███░░███  ███░░███ ███░░███ ░███░░███  ░░░░░███░░░███░
     ░███ ░███   ███████  ░███ ░███ ░███ ░███░███ ░░░  ░███ ░███   ███████  ░███
     ░███ ░███  ███░░███  ░███ ░███ ░███ ░███░███  ███ ░███ ░███  ███░░███  ░███ ███
     ████ █████░░████████ ████ █████░░██████ ░░██████  ████ █████░░███████  ░░█████
    ░░░░ ░░░░░  ░░░░░░░░ ░░░░ ░░░░░  ░░░░░░   ░░░░░░  ░░░░ ░░░░░  ░░░░░░░░   ░░░░░
    
Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/ss/nanochat/scripts/base_train.py", line 82, in <module>
    assert args.target_param_data_ratio > 0 or args.target_param_data_ratio == -1, "target-param-data-ratio must be positive (or -1 to disable)"
AssertionError: target-param-data-ratio must be positive (or -1 to disable)
(nanochat) ss@ss-Predator-PH315-53:~/nanochat$

[suraj-self]

Hi @svlandeg
I've implemented the fix using an assert to fail fast when 0 is provided, as it prevents the ZeroDivisionError and clarifies that -1 should be used to disable scaling.

However, would you prefer the script to gracefully fall back to the default Chinchilla ratio (10.5) when 0 is detected instead of crashing?