refactor: Add ProviderContext for a flexible storage directory

- Introduce ProviderContext class to decouple provider storage paths from absolute paths
- Add storage_dir attribute to StackRunConfig to accept CLI options
- Implement storage directory resolution with prioritized fallbacks:
  1. CLI option (--state-directory)
  2. Environment variable (LLAMA_STACK_STATE_DIR)
  3. Default distribution directory
- Standardize provider signatures to follow context, config, deps pattern
- Update provider implementations to use the new context-based approach
- Add comprehensive tests to verify state directory resolution

llama_stack/providers/inline/post_training/torchtune/__init__.py

@@ -7,6 +7,7 @@
 from typing import Any
 
 from llama_stack.distribution.datatypes import Api
+from llama_stack.providers.datatypes import ProviderContext
 
 from .config import TorchtunePostTrainingConfig
 
@@ -14,12 +15,14 @@ from .config import TorchtunePostTrainingConfig
 
 
 async def get_provider_impl(
+    context: ProviderContext,
     config: TorchtunePostTrainingConfig,
     deps: dict[Api, Any],
 ):
     from .post_training import TorchtunePostTrainingImpl
 
     impl = TorchtunePostTrainingImpl(
+        context,
         config,
         deps[Api.datasetio],
         deps[Api.datasets],

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

@@ -20,6 +20,7 @@ from llama_stack.apis.post_training import (
     PostTrainingJobStatusResponse,
     TrainingConfig,
 )
+from llama_stack.providers.datatypes import ProviderContext
 from llama_stack.providers.inline.post_training.torchtune.config import (
     TorchtunePostTrainingConfig,
 )
@@ -42,10 +43,12 @@ _JOB_TYPE_SUPERVISED_FINE_TUNE = "supervised-fine-tune"
 class TorchtunePostTrainingImpl:
     def __init__(
         self,
+        context: ProviderContext,
         config: TorchtunePostTrainingConfig,
         datasetio_api: DatasetIO,
         datasets: Datasets,
     ) -> None:
+        self.context = context
         self.config = config
         self.datasetio_api = datasetio_api
         self.datasets_api = datasets