[Post training] make validation steps configurable (#715)

## what does this PR do? 
The current code hardcode the validation steps to run (forgot to change
it after testing). in this PR, we make it configurable by training
config

## test 
On client side, issue a post training request with 20 validation steps,
server side logging shows that it runs 20 validation steps successfully
<img width="1128" alt="Screenshot 2025-01-02 at 8 21 06 PM"
src="https://github.com/user-attachments/assets/7a757516-c6ba-41d4-85c5-361a80ecf46e"
/>

llama_stack/apis/post_training/post_training.py

@@ -58,6 +58,7 @@ class TrainingConfig(BaseModel):
     n_epochs: int
     max_steps_per_epoch: int
     gradient_accumulation_steps: int
+    max_validation_steps: int
     data_config: DataConfig
     optimizer_config: OptimizerConfig
     efficiency_config: Optional[EfficiencyConfig] = None

llama_stack/providers/inline/post_training/torchtune/recipes/lora_finetuning_single_device.py

@@ -137,6 +137,7 @@ class LoraFinetuningSingleDevice:
         self.global_step = 0
 
         self._gradient_accumulation_steps = training_config.gradient_accumulation_steps
+        self.max_validation_steps = training_config.max_validation_steps
 
         self._clip_grad_norm = 1.0
         self._enable_activation_checkpointing = (
@@ -583,7 +584,7 @@ class LoraFinetuningSingleDevice:
         log.info("Starting validation...")
         pbar = tqdm(total=len(self._validation_dataloader))
         for idx, batch in enumerate(self._validation_dataloader):
-            if idx == 10:
+            if idx == self.max_validation_steps:
                 break
             torchtune_utils.batch_to_device(batch, self._device)