Add _core._get_backend_details() utility for testing and debugging

[NicolasHug]

This PR adds a _core utility to get details about the current backend / interface being used. It reports:

• the name of the interface (CPU, FFmpeg CUDA, Beta CUDA)
• whether a CPU fallback is happening for FFmpeg CUDA and Beta CUDA.

This utility is needed for testing, see resolved TODO in our tests. I also expect it will also be useful for debugging purposes, e.g. when working on #943. We might want to eventually expose it publicly, so that users know whether a fallback is happening or not.

ATM, the returned value is a string with no particular structure. I think that's fine for now. Everything is private. We can think of defining something more structured later (keeping in mind that our custom ops don't have much flexibility in terms of what can be returned).

The new functionality is exposed as:

• _core._get_backend_details(decoder: torch.Tensor) -> str
• std::string SingleStreamDecoder::getInterfaceDetails()
• std::string DeviceInterface::getDetails()

Happy to bikeshed on the names.

[Dan-Flores]

Looks good! Maybe we should add device to the function names, to be clearer on what they get?
getDetails -> getDeviceDetails
getInterfaceDetails -> getDeviceInterfaceDetails

[scotts]

There's at least one other thing we may want to expose such a private function for for testing: color space library. We have tests that exercise the different color space libraries, but we can't know for sure from the outside which one we're using.