[PyTorch Mobile] Outline DispatchStub::get_call_ptr() (#51908)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51908
As suggested by swolchok. The idea is to outline `DispatchStub::get_call_ptr()` and also not have it specialized per instantiation of the `DispatchStub` class since it results in size bloat for mobile.
ghstack-source-id: 121873712
Test Plan:
Build + Circle CI.
### lightspeed
```
D26324255-V8 (https://www.internalfb.com/intern/diff/D26324255/?dest_number=121462800)
messenger-experimental-optimized-device: Succeeded
Change in Download Size for arm64 + 3x assets variation: -13.0 KiB
Change in Uncompressed Size for arm64 + 3x assets variation: -51.4 KiB
Mbex Comparison: https://our.intern.facebook.com/intern/mbex/bsb:325359338899206@base/bsb:325359338899206@diff/
```
### igios
```
D26324255-V8 (https://www.internalfb.com/intern/diff/D26324255/?dest_number=121462800)
igios: Succeeded
Change in Download Size for arm64 + 3x assets variation: -9.2 KiB
Change in Uncompressed Size for arm64 + 3x assets variation: -23.4 KiB
Mbex Comparison: https://our.intern.facebook.com/intern/mbex/bsb:823799391811488@base/bsb:823799391811488@diff/
```
### fbios-pika
```
D26324255-V8 (https://www.internalfb.com/intern/diff/D26324255/?dest_number=121462800)
fbios-pika: Succeeded
Change in Download Size for arm64 + 3x assets variation: -8.0 KiB
Change in Uncompressed Size for arm64 + 3x assets variation: -22.7 KiB
Mbex Comparison: https://our.intern.facebook.com/intern/mbex/bsb:1345469719167068@base/bsb:1345469719167068@diff/
```
Reviewed By: swolchok
Differential Revision: D26324255
fbshipit-source-id: 61aba8687f4c1b742fa9d9d917a026abc8d9c328
