Dispatch mode and torch function disable are inconsistent

[ezyang]

🐛 Describe the bug

no_dispatch disables both mode and subclass (because it excludes the Python dispatch key, which controls for both)

_C.DisableTorchFunction only disables subclass (this behavior is explicitly tested for in test_mode_notimplemented_loop
and test_disable_subclass_not_mode)

Probably easiest fix is to rename DisableTorchFunction to DisableTorchFunctionSubclass and then add a mode disabler.

Versions

master

cc @hameerabbasi @rgommers @peterbell10 @Chillee @zou3519 @albanD @samdow

[ezyang]

cc @samdow