feat: pod overlay for kubernetes scheduler (#1067,#1068) (#1148)

Co-authored-by: Alexander Zhipa <azzhipa@amazon.com>

Author: clumsy

torchx/schedulers/kubernetes_scheduler.py

@@ -27,6 +27,76 @@
 See the
 `Volcano Quickstart <https://github.com/volcano-sh/volcano>`_
 for more information.
+
+Pod Overlay
+===========
+
+You can overlay arbitrary Kubernetes Pod fields on generated pods by setting
+the ``kubernetes`` metadata on your role. The value can be:
+
+- A dict with the overlay structure
+- A resource URI pointing to a YAML file (e.g. ``file://``, ``s3://``, ``gs://``)
+
+Merge semantics:
+- **dict**: recursive merge (upsert)
+- **list**: append by default, replace if tuple (Python) or ``!!python/tuple`` tag (YAML)
+- **primitives**: replace
+
+.. code:: python
+
+    from torchx.specs import Role
+
+    # Dict overlay - lists append, tuples replace
+    role = Role(
+        name="trainer",
+        image="my-image:latest",
+        entrypoint="train.py",
+        metadata={
+            "kubernetes": {
+                "spec": {
+                    "nodeSelector": {"gpu": "true"},
+                    "tolerations": [{"key": "nvidia.com/gpu", "operator": "Exists"}],  # appends
+                    "volumes": ({"name": "my-volume", "emptyDir": {}},)  # replaces
+                }
+            }
+        }
+    )
+
+    # File URI overlay
+    role = Role(
+        name="trainer",
+        image="my-image:latest",
+        entrypoint="train.py",
+        metadata={
+            "kubernetes": "file:///path/to/pod_overlay.yaml"
+        }
+    )
+
+CLI usage with builtin components:
+
+.. code:: bash
+
+    $ torchx run --scheduler kubernetes dist.ddp \\
+        --metadata kubernetes=file:///path/to/pod_overlay.yaml \\
+        --script train.py
+
+Example ``pod_overlay.yaml``:
+
+.. code:: yaml
+
+    spec:
+      nodeSelector:
+        node.kubernetes.io/instance-type: p4d.24xlarge
+      tolerations:
+        - key: nvidia.com/gpu
+          operator: Exists
+          effect: NoSchedule
+      volumes: !!python/tuple
+        - name: my-volume
+          emptyDir: {}
+
+The overlay is deep-merged with the generated pod, preserving existing fields
+and adding or overriding specified ones.
 """
 
 import json
@@ -45,6 +115,7 @@
     Tuple,
     TYPE_CHECKING,
     TypedDict,
+    Union,
 )
 
 import torchx
@@ -97,6 +168,40 @@
 RESERVED_MILLICPU = 100
 RESERVED_MEMMB = 1024
 
+
+def _apply_pod_overlay(pod: "V1Pod", overlay: Dict[str, Any]) -> None:
+    """Apply overlay dict to V1Pod object, merging nested fields.
+
+    Merge semantics:
+    - dict: upsert (recursive merge)
+    - list: append by default, replace if tuple
+    - primitives: replace
+    """
+    from kubernetes import client
+
+    api = client.ApiClient()
+    pod_dict = api.sanitize_for_serialization(pod)
+
+    def deep_merge(base: Dict[str, Any], overlay: Dict[str, Any]) -> None:
+        for key, value in overlay.items():
+            if isinstance(value, dict) and key in base and isinstance(base[key], dict):
+                deep_merge(base[key], value)
+            elif isinstance(value, tuple):
+                base[key] = list(value)
+            elif (
+                isinstance(value, list) and key in base and isinstance(base[key], list)
+            ):
+                base[key].extend(value)
+            else:
+                base[key] = value
+
+    deep_merge(pod_dict, overlay)
+
+    merged_pod = api._ApiClient__deserialize(pod_dict, "V1Pod")
+    pod.spec = merged_pod.spec
+    pod.metadata = merged_pod.metadata
+
+
 RETRY_POLICIES: Mapping[str, Iterable[Mapping[str, str]]] = {
     RetryPolicy.REPLICA: [],
     RetryPolicy.APPLICATION: [
@@ -402,6 +507,17 @@ def app_to_resource(
             replica_role.env["TORCHX_IMAGE"] = replica_role.image
 
             pod = role_to_pod(name, replica_role, service_account)
+            if k8s_metadata := role.metadata.get("kubernetes"):
+                if isinstance(k8s_metadata, str):
+                    import fsspec
+
+                    with fsspec.open(k8s_metadata, "r") as f:
+                        k8s_metadata = yaml.unsafe_load(f)
+                elif not isinstance(k8s_metadata, dict):
+                    raise ValueError(
+                        f"metadata['kubernetes'] must be a dict or resource URI, got {type(k8s_metadata)}"
+                    )
+                _apply_pod_overlay(pod, k8s_metadata)
             pod.metadata.labels.update(
                 pod_labels(
                     app=app,
@@ -637,7 +753,7 @@ def schedule(self, dryrun_info: AppDryRunInfo[KubernetesJob]) -> str:
             else:
                 raise
 
-        return f'{namespace}:{resp["metadata"]["name"]}'
+        return f"{namespace}:{resp['metadata']['name']}"
 
     def _submit_dryrun(
         self, app: AppDef, cfg: KubernetesOpts