[torch] Remove migration warning for ScriptDict
Summary:
This commit removes the warning that suggests that users script their
dictionaries before passing them into TorchScript code. The ScriptDict feature
is not fully ready, so it does not make sense to recommend this yet.
Test Plan:
Sandcastle.
In addition, the PyPER test broken by the original diff passes:
```
buck test mode/opt //caffe2/torch/fb/training_toolkit/backend/tests:test_model_materializer_full_sync_lwt -- --exact 'caffe2/torch/fb/training_toolkit/backend/tests:test_model_materializer_full_sync_lwt - caffe2.torch.fb.training_toolkit.backend.tests.test_model_materializer_full_sync_lwt.ModelMaterializerFullSyncLwtTest: test_materialization_determinism_cpu' --run-disabled
```
Differential Revision: D28891351
fbshipit-source-id: 2a3a00cde935d670fb1dc7fd8c709ae9c2ad8cdc
