ATen | Fix potential crash if `MTLCreateSystemDefaultDevice` return nil (#66859)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66859
`MTLCreateSystemDefaultDevice` can return `nil`. If that happens then inside `createDeviceInfo`, we'll crash trying to convert the `nullptr` from `device.name.UTF8String` into a `std::string`.
Let's fix it by returning early in setup if there's no Metal device. But also make `createDeviceInfo` safe if we do pass in `nil`.
Test Plan: * CircleCI
Reviewed By: xta0
Differential Revision: D31759690
fbshipit-source-id: 74e878ab5b8611250c4843260f1d2e4eab22cdaf
