Make Phi4MMForCausalLM.forward's num_logits_to_keep actually optional

Fixes:
```
File "/home/phh/.cache/huggingface/modules/transformers_modules/microsoft/Phi-4-multimodal-instruct/a11e830f953efbce02a57d3b672f84c5140e3f94/modeling_phi4mm.py", line 2137, in forward
logits = self.lm_head(hidden_states[:, -num_logits_to_keep:, :])
^^^^^^^^^^^^^^^^^^^
TypeError: bad operand type for unary -: 'NoneType'
```

modeling_phi4mm.py

@@ -2134,7 +2134,10 @@ class Phi4MMForCausalLM(Phi4MMPreTrainedModel, GenerationMixin):
 
         hidden_states = outputs[0]
         # Only compute necessary logits, and do not upcast them to float if we are not computing the loss
-        logits = self.lm_head(hidden_states[:, -num_logits_to_keep:, :])
+        if num_logits_to_keep:
+            logits = self.lm_head(hidden_states[:, -num_logits_to_keep:, :])
+        else:
+            logits = self.lm_head(hidden_states)
 
         loss = None
         if labels is not None: