Fix num_train_epochs=None causing TypeError in GRPOConfig (#3972)

When users pass `num_train_epochs=None` to GRPOConfig (relying on
max_steps to control training duration), Trainer.__init__ fails with:

  TypeError: '>' not supported between instances of 'NoneType' and 'int'

This happens because transformers.Trainer does `args.num_train_epochs > 0`
in its __init__ which fails when the value is None.

This fix converts None to 3.0 (the default) before Trainer initialization.
The actual training duration is still controlled by max_steps since it
takes precedence when both are set.

Example that now works:
```python
config = GRPOConfig(
    num_train_epochs=None,  # Previously caused TypeError
    max_steps=500,          # This controls actual duration
    ...
)
```

unsloth/models/rl.py

@@ -891,6 +891,15 @@ def _patch_trl_rl_trainers(trainer_file = "grpo_trainer"):
         )
         extra_args += learning_rate_check
 
+    # Fix num_train_epochs = None causing TypeError in Trainer.__init__
+    # Trainer does `args.num_train_epochs > 0` which fails when None
+    if "num_train_epochs" in call_args:
+        num_train_epochs_check = (
+            "if num_train_epochs is None:\n"
+            "    num_train_epochs = 3.0  # Default to 3 epochs if None, max_steps will override\n"
+        )
+        extra_args += num_train_epochs_check
+
     # Check if max_seq_length is NOT defined (max_length is now default)
     if "max_seq_length" not in call_args and "max_length" in call_args:
         max_seq_length_pre = """max_seq_length : Optional[int] = field(