Test cases for backend fallback kernels

[smessmer]

Stack from ghstack:

• [wip] Use codegen'ed unboxing wrappers #30370 [wip] Use codegen'ed unboxing wrappers
• Boxed variable dispatch #29934 Boxed variable dispatch
• backend fallback test #29682 backend fallback test
• better boxing #29681 better boxing
• Hide the OperatorKernel* argument from the stack based kernel API #29337 Hide the OperatorKernel* argument from the stack based kernel API
• OperatorHandle::callBoxed/callUnboxed #29330 OperatorHandle::callBoxed/callUnboxed
• Test cases for backend fallback kernels #29214 Test cases for backend fallback kernels
• Add a OperatorHandle argument to boxed kernels #29201 Add a OperatorHandle argument to boxed kernels
• Make Dispatcher::backendFallbackKernels_ an array #30340 Make Dispatcher::backendFallbackKernels_ an array
• [pytorch][PR] Remove LeftRight from OperatorEntry and DispatchTable. #30333 [pytorch][PR] Remove LeftRight from OperatorEntry and DispatchTable.
• [pytorch][PR] Convert KernelTable to a flat-indexed array rather than a hashtable. #30332 [pytorch][PR] Convert KernelTable to a flat-indexed array rather than a hashtable.

Differential Revision: D18329308

[ezyang]

These tests are fine but they are not the same as the old boxed fallback tests, which showed you how to use backend fallback to do something more similar to what you /actually/ want to do (fallthrough to the underlying kernel, and do some other stuff along the way.)

[ezyang]

(So, I'm not blocking landing this PR, but I don't think @mcarilli will really be satisfied by these tests.)

[smessmer]

@ezyang @mcarilli added those ones in #29682