feat: removing hard limits on learning rate

[ysjprojects]

Addresses #3230

Users can now bypass the hard limit by setting UNSLOTH_USE_STRICT_MODE="1".

The error message will be printed to the console as a warning instead.

Note: for backward compatibility, hard limits on learning rate is still imposed by default:

use_strict_mode = os.environ.get('UNSLOTH_USE_STRICT_MODE', '1') == '1'