[pytorch][mobile] make sure mobile build work with dynamic dispatch (#34038)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34038
Mobile build doesn't include autograd/VariableType dispatch. As the
result AutoNonVariableTypeMode needs to be set in mobile runtime.
With static dispatch this works is done inside generated jit-dispatch
code - AutoNonVariableTypeMode needs to be set on per-op basis. Setting
it globally or setting it for wrong ops might break some `is_variable()`
checks in the codebase.
Thanks to the unification of Variable class and Tensor class, all
is_variable() checks have been removed, so AutoNonVariableTypeMode can
be set globally now.
We never tested inference-only mobile build with dynamic dispatch. It
seems that dynamic dispatch also requires setting AutoNonVariableTypeMode
for our mobile build (where VariableType functions are not registered).
Verified the end-to-end test works with this change:
```
TEST_CUSTOM_BUILD_DYNAMIC=1 test/mobile/custom_build/build.sh
```
Test Plan: Imported from OSS
Differential Revision: D20193329
Pulled By: ljk53
fbshipit-source-id: cc98414d89d12463dc82b0cdde0b6160dafc0349
