Add Autocast Support for FakeTensors / use fake device dispatch keys (#82449)
From PR:
```
Note: [Fake Tensor Dispatch Keys]
In order to model the behavior of device-specific autocast
and autograd logic, we update the dispatch keys of FakeTensors
to reflect their fake device. This includes the BackendComponent
(DispatchKey::Meta -> DispatchKey::CUDA), and also the BackendComponent
related Autocast and Autograd keys. __torch__dispatch__ sits below
Autocast and Autograd, and is only invoked when we are at the
kernel for the BackendComponent. Then, we add Meta to the
thread-local dispatch include set to hit the meta kernel
instead of the kernel of the BackendComponent for the fake device.
```
Also adds the `conv1/2/3d.padding` operators to the Autocast rule set. Without that fix, the FakeTensor dtype would diverge.
See: https://github.com/pytorch/pytorch/issues/81608
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82449
Approved by: https://github.com/ezyang
