[2/n][torchtune integration] implement job management and return training artifacts (#593)

### Context 
In this PR, we 
- Implement the post training job management and get training artifacts
apis
  - get_training_jobs
  - get_training_job_status
  - get_training_job_artifacts
- get_training_job_logstream is deleted since the trace can be directly
accessed by UI with Jaeger
https://llama-stack.readthedocs.io/en/latest/building_applications/telemetry.html#jaeger-to-visualize-traces
- Refactor the post training and training types definition to make them
more intuitive.
- Rewrite the checkpointer to make it compatible with llama-stack file
system and can be recognized during inference


### Test
Unit test
`pytest llama_stack/providers/tests/post_training/test_post_training.py
-m "torchtune_post_training_huggingface_datasetio" -v -s --tb=short
--disable-warnings`

<img width="1506" alt="Screenshot 2024-12-10 at 4 06 17 PM"
src="https://github.com/user-attachments/assets/16225029-bdb7-48c4-9d13-e580cc769c0a">


e2e test with client side call

<img width="888" alt="Screenshot 2024-12-10 at 4 09 44 PM"
src="https://github.com/user-attachments/assets/de375e4c-ef67-4dcc-a045-4037d9489191">

llama_stack/providers/tests/post_training/test_post_training.py

@@ -19,6 +19,7 @@ class TestPostTraining:
     @pytest.mark.asyncio
     async def test_supervised_fine_tune(self, post_training_stack):
         algorithm_config = LoraFinetuningConfig(
+            type="LoRA",
             lora_attn_modules=["q_proj", "v_proj", "output_proj"],
             apply_lora_to_mlp=True,
             apply_lora_to_output=False,
@@ -59,3 +60,33 @@ class TestPostTraining:
         )
         assert isinstance(response, PostTrainingJob)
         assert response.job_uuid == "1234"
+
+    @pytest.mark.asyncio
+    async def test_get_training_jobs(self, post_training_stack):
+        post_training_impl = post_training_stack
+        jobs_list = await post_training_impl.get_training_jobs()
+        assert isinstance(jobs_list, List)
+        assert jobs_list[0].job_uuid == "1234"
+
+    @pytest.mark.asyncio
+    async def test_get_training_job_status(self, post_training_stack):
+        post_training_impl = post_training_stack
+        job_status = await post_training_impl.get_training_job_status("1234")
+        assert isinstance(job_status, PostTrainingJobStatusResponse)
+        assert job_status.job_uuid == "1234"
+        assert job_status.status == JobStatus.completed
+        assert isinstance(job_status.checkpoints[0], Checkpoint)
+
+    @pytest.mark.asyncio
+    async def test_get_training_job_artifacts(self, post_training_stack):
+        post_training_impl = post_training_stack
+        job_artifacts = await post_training_impl.get_training_job_artifacts("1234")
+        assert isinstance(job_artifacts, PostTrainingJobArtifactsResponse)
+        assert job_artifacts.job_uuid == "1234"
+        assert isinstance(job_artifacts.checkpoints[0], Checkpoint)
+        assert job_artifacts.checkpoints[0].identifier == "Llama3.2-3B-Instruct-sft-0"
+        assert job_artifacts.checkpoints[0].epoch == 0
+        assert (
+            "/.llama/checkpoints/Llama3.2-3B-Instruct-sft-0"
+            in job_artifacts.checkpoints[0].path
+        )