Increased compile time max GPUs to 512. Switched to int16_t DeviceIndex. (#119639)
Fixes #115331.
This PR increases the number of valid GPU devices to 512 (from 64) in order to future-proof PyTorch for providers that offer [single nodes with a large device count](https://www.tensorwave.com/). Until now, `DeviceIndex` was an `int8_t`, thus multiple changes were necessary:
- `DeviceIndex` changed to `int16_t`. Updated consumers that assume it to be an `int8_t`.
- Updated bounds checking for `torch.device()` in the Python frontend. Right now, we allow funny things like `torch.device('cpu', 200).index == -56`, which is undefined behavior. I inserted some checks to only allow values between 0 and `c10::Device::MAX_NUM_DEVICES - 1`.
- Updated the `ArgumentInfo` struct as it hardcodes the device index as 8 bit field [^1]. Might be a breaking change, not sure if users rely on this.
- Introduced `c10::Device::MAX_NUM_DEVICES` as a replacement for the old `C10_COMPILE_TIME_MAX_GPUS`
[^1]: This field was unsigned, so I guess this has also been undef behavior the whole time? Our default device index is -1, so this always wrapped around to 255 when written to the `ArgumentInfo` struct. When I switched the `DeviceIndex` to `int16_t`, it actually stayed 255 after unpacking from `ArgumentInfo` again, as the `DeviceIndex` was now wide enough that it didn't wrap back to -1.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119639
Approved by: https://github.com/cyyever, https://github.com/albanD
