SFTTrainer: Logging Experts Statistics

### Feature request

Hello,

When fine-tuning MoE models and setting `output_router_logits=True`, I’ve found it useful to report per-expert token throughput statistics. This information is extremely valuable for identifying which expert layers are most active and therefore the best candidates for adding LoRA adapters later on. I implemented this functionality myself, and although it isn’t fully optimized, it has worked well for my use case.

```python
class SFTTrainerWithExpertsLogging(SFTTrainer):
def compute_loss(
self,
model: nn.Module,
inputs: dict[str, torch.Tensor | Any],
return_outputs: bool = False,
num_items_in_batch: torch.Tensor | None = None,
):
(loss, outputs) = super().compute_loss(
model, inputs, return_outputs=True, num_items_in_batch=num_items_in_batch
)

mode = "train" if self.model.training else "eval"

# Compute expert usage statistics
if hasattr(outputs, "router_logits") and outputs.router_logits is not None:
labels = inputs.get("labels")
mask = labels != -100 if labels is not None else None

with torch.no_grad():
for layer_idx, router_logit in enumerate(outputs.router_logits):
expert_indices = router_logit.argmax(dim=-1)
expert_indices = expert_indices.flatten()[mask.flatten()]
num_experts = router_logit.shape[-1]

# Count tokens per expert locally
expert_counts = torch.bincount(expert_indices, minlength=num_experts)[:num_experts]

# Sum across all processes
if self.accelerator.use_distributed:
torch.distributed.all_reduce(expert_counts, op=torch.distributed.ReduceOp.SUM)

# Normalize
expert_percentages = expert_counts / num_items_in_batch
for expert_idx in range(num_experts):
metric_key = f"expert_usage_layer_{layer_idx}_expert_{expert_idx}"
self._metrics[mode][metric_key].append(expert_percentages[expert_idx].item())

if hasattr(outputs, "aux_loss") and outputs.aux_loss is not None:
aux_loss = self.accelerator.gather_for_metrics(outputs.aux_loss).mean().item()
self._metrics[mode]["aux_loss"].append(aux_loss)

return (loss, outputs) if return_outputs else loss
```

### Motivation

When fine-tuning MoE models, it’s often inefficient to attach LoRA adapters to every expert layer. By collecting statistics on which experts are actually being activated, we can make much better use of PEFT methods and selectively target only the most impactful experts.

### Your contribution

see code above.