Don't use NonVariableTypeMode in custom ops

Potentially fixes #37306

Differential Revision: [D21261946](https://our.internmc.facebook.com/intern/diff/D21261946/)

[ghstack-poisoned]

aten/src/ATen/core/VariableFallbackKernel.cpp

@@ -28,47 +28,8 @@ using c10::KernelFunction;
 
 namespace {
 
-void variable_fallback_kernel(const OperatorHandle& op, Stack* stack) {
-    at::AutoNonVariableTypeMode _var_guard(true);
-    op.callBoxed(stack);
-}
-
 TORCH_LIBRARY_IMPL(_, Autograd, m) {
-#ifdef C10_MOBILE
-  // As custom mobile build might not include variable kernels, we need
-  // leverage variable fallback mechanism as well. The goals are:
-  // 1) don't break forward pass for inference-only mobile build;
-  // 2) don't break forward/backward pass for mobile build with necessary
-  // variable kernels registered;
-  //
-  // This `fallthrough` kernel is for #1 - because not all kernels support
-  // boxed call yet, registering `variable_fallback_kernel` might fail.
-  // When an op has variable kernel registered explicitly dispatcher will
-  // call it instead of `fallthrough`, so `fallthrough` won't break
-  // dispatching to real variable kernels for case #2.
-  //
-  // The substantial difference between fallback and fallthrough is whether
-  // AutoNonVariableTypeMode guard is applied. There are two downstream
-  // effects of the guard:
-  // a) stop calling variable kernels of other ops called by the current op;
-  //    For case #1, there is no difference because no variable kernels are
-  //    registered. For case #2, there is no difference as long as ALL used
-  //    ops have real variable kernels registered, where the guard will be
-  //    set properly in real variable kernels. There is potential issue only
-  //    when variable kernels are partially registered for used ops.
-  // b) `variable_excluded_from_dispatch()` method returns the state of the
-  //    NonVariableTypeMode. As of when this diff is written, the callers of
-  //    the method are ALL asserting it returns true; the only exception is
-  //    the deprecated `is_variable()` method. So we make the method to always
-  //    return true for mobile builds. It shouldn't break case #1/#2 as long
-  //    as `is_variable()` is not used.
-  //
-  // We can remove this `fallthrough` kernel when all kernels support boxed
-  // call.
   m.fallback(torch::CppFunction::makeFallthrough());
-#else
-  m.fallback(torch::CppFunction::makeFromBoxedFunction<&variable_fallback_kernel>());
-#endif
 }
 
 }