Support Jetter (buck) workspace

Summary:
### Support Jetter (buck) Workspace

Internal changes to support Jetter workspace, which builds workspace patches via Jetter at submit time.

#### Key Changes:

* Modified `mast.py` and `msl.py` to remove user-facing examples and notes.
* Added `Fbsource` to `__init__.py` and updated `__all__` to include it.
* Updated `hpc_scheduler.py` to include JetterWorkspace and create_jetter_scheduler.
* Added a new example in `launch.py` demonstrating how to submit a BUCK-based job using JetterWorkspace.

These changes lay the groundwork for supporting Jetter workspaces in TorchX, enabling more efficient and flexible job submissions.

Differential Revision: D92295898

Author: kiukchung

.claude/rules/coding-conventions.md

@@ -103,3 +103,22 @@ raise ValueError(f"unknown scheduler: `{scheduler}`")
 ## Tests
 
 Location: `module/test/module_test.py` | Class: `ModuleTest` | Method: `test_*`
+
+## Runopts (Scheduler Configuration)
+
+**Avoid adding runopts to implement features.** This should be a last resort.
+
+**Why:**
+1. Runopts reduce AppDef portability—the AppDef becomes tightly coupled to specific schedulers
+2. Runopts obscure scheduler behavior, making it hard for users to reason about the resulting job
+
+**Preferred alternatives:**
+- **Overlays** in AppDef or Role metadata (search scheduler source for logic that applies user-specified overrides on top of the generated request)
+- **Workspace type checking**: `isinstance(self, SomeWorkspace)`
+- **Scheduler/workspace composition**: Compose schedulers and workspaces to encapsulate behavior
+- **Extend core APIs**: Add attributes/methods to `torchx.specs`, `torchx.schedulers`, `torchx.workspace`, or `torchx.runner`
+
+**Anti-patterns (avoid these):**
+- **Resource capabilities**: `resource.capabilities["key"] = "value"` — overloads an unrelated field
+
+Only add runopts when no alternative exists and the option is fundamental to scheduler operation.

torchx/runner/api.py

@@ -88,8 +88,8 @@ class Runner:
 
     def __init__(
         self,
-        name: str,
-        scheduler_factories: dict[str, SchedulerFactory],
+        name: str = "",  # session names can be empty
+        scheduler_factories: dict[str, SchedulerFactory] | None = None,
         component_defaults: dict[str, dict[str, str]] | None = None,
         scheduler_params: dict[str, object] | None = None,
     ) -> None:
@@ -102,7 +102,9 @@ def __init__(
             schedulers: a list of schedulers the runner can use.
         """
         self._name: str = name
-        self._scheduler_factories = scheduler_factories
+        self._scheduler_factories: dict[str, SchedulerFactory] = (
+            scheduler_factories or {}
+        )
         self._scheduler_params: dict[str, Any] = {
             **(self._get_scheduler_params_from_env()),
             **(scheduler_params or {}),