[caffe2] fix type annotations for workspace.SwitchWorkspace() (#77464)
Summary: The `create_if_missing` parameter is optional, and defaults to `None`.
Test Plan:
Confirmed that Pyre no longer complains about calling `SwitchWorkspace` with a
single string argument.
Differential Revision: D36366987
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77464
Approved by: https://github.com/voznesenskym
