Make static dispatch turn off variable before entering the kernel. (#26908)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26908

This is because in static dispatch, variable is not supported.  But
you might still "have" a variable somehow (e.g., in JIT initialization
code), so we need to translate between the two worlds.

Attempt to fix #26764

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: Imported from OSS

Differential Revision: D17607550

Pulled By: ezyang

fbshipit-source-id: ea2cb931215d749efd95259ad7a1a8ae96c38aed

aten/src/ATen/function_wrapper.py

@@ -239,10 +239,17 @@ static inline ${return_type} ${api_name}(${formals}) {
 
 # In order to rely on the linker to strip unused ops, it requires us to dispatch statically
 # in Functions.h and TensorMethods.h.
+#
+# NB: The default body also needs to apply a variable guard, as in some
+# situations what we think is a default body actually does have an
+# explicit derivative, and thereby would have gotten unwrapped by
+# the time you get to the implementation.
 STATIC_DISPATCH_FUNCTION_DEFAULT_BODY = CodeTemplate("""\
+at::AutoNonVariableTypeMode _var_guard(true);
 ${return_call} TypeDefault::${native_type_method_dispatch}(${native_arguments});
 """)
 STATIC_DISPATCH_FUNCTION_SWITCH_BODY = CodeTemplate("""\
+at::AutoNonVariableTypeMode _var_guard(true);
 switch(tensorTypeIdToBackend(impl::dispatchTypeId(${type_set}))) {
     ${static_dispatch_function_switches}
     default:

aten/src/ATen/templates/TensorMethods.h

@@ -11,6 +11,7 @@
 #include <ATen/core/dispatch/Dispatcher.h>
 #include <ATen/core/NamedTensor.h>
 #include <ATen/core/EnableNamedTensor.h>
+#include <ATen/core/LegacyTypeDispatch.h>
 
 #ifdef USE_STATIC_DISPATCH
 #include <ATen/TypeDefault.h>