(torchx/specs) Remove RunConfig in favor of using Dict[str, CfgVal] directly (#341)

Summary:
Pull Request resolved: pytorch/torchx#341

Pull Request resolved: #163

Closes: pytorch/torchx#302

`RunConfig` was a redundant wrapper around a python `Dict[str, CfgVal]`. This diff simply removes the unnecessary `RunConfig` class in favor of using `Dict[str, CfgVal]` directly. Makes client-side code much nicer since:

```
# BEFORE
cfg = specs.RunConfig()
cfg.set("key0", "val")
cfg.set("key1", "val")
...

# AFTER
cfg = {"key0": "val", "key1": "val"}
```

Note that certain APIs are typed as `Mapping[str, CfgVal]` to deal with contravariance for container types in pyre.

Reviewed By: d4l3k

Differential Revision: D32400298

fbshipit-source-id: 5cb9387d28541a39ad2b9bbdb7232196ce8ce82e

Author: Kiuk Chung

torchelastic/tsm/driver/__init__.py

@@ -70,9 +70,9 @@
 from torchx.schedulers import get_schedulers
 from torchx.schedulers.api import DescribeAppResponse, Scheduler  # noqa: F401 F403
 from torchx.specs.api import (  # noqa: F401 F403
+    AppDef as Application,
     AppDryRunInfo,
     AppHandle,
-    AppDef as Application,
     AppState,
     AppStatus,
     ReplicaState,
@@ -81,7 +81,6 @@
     RetryPolicy,
     Role,
     RoleStatus,
-    RunConfig,
     SchedulerBackend,
     is_terminal,
     macros,