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.

re-export of #29902

Differential Revision: D18660421